Ever notice that a single hotel can be rated three, four and even five stars?

That's the case with Singapore's iconic Marina Bay Sands, which is rated five stars on Booking.com, four stars on Forbes Travel Guide and three stars on Frommer's.

This is because star ratings are "not regulated in a consistent way across the industry," said Yngvar Stray, the general manager of Capella Singapore.

Booking.com said its stars are designated by the hotels themselves. Forbes Travel Guide, however, uses professional, anonymous inspectors who access "900 objective stands," according to its website. And to add to the confusion, Frommer's website states its star ratings only go from one ("recommended") to three stars ("exceptional").

The sheer number of companies that award stars isn't helping.

Decades ago, few companies handed out stars — or diamonds, as the American Automobile Association calls its ratings. But now, countless magazines, guidebooks and websites issue them.

And in places like Australia, India and Dubai, hotels are rated by governments, tourism boards and industry councils.

Can the stars be aligned?

Some organizations are certainly trying.

The non-profit organization Hotelstars Union uses common criteria to unify star ratings across Europe. So far, 20 countries have signed on, and more than 22,000 hotels have been classified.

But Europe's most popular countries to visit — France, Spain and Italy — are not among the member countries. Each has its own rating systems, which are run by national or regional governments.

In the United Kingdom, the AA motoring association and the tourism board Visit England award stars using the same criteria. They also display their ratings on one website (RatedTrips.com). 

A project called "World Hotel Rating" was meant to standardize hotel ratings around the world. But that was more than a decade ago, and nothing has materialized yet. 

World Hotel Rating did not respond to CNBC requests for comment.

Do hotel stars still matter?

They do, according to Tim Hentschel, CEO of the travel technology company HotelPlanner.

"We actually find star ratings are quite valuable to our customers," he said. "When you go in to pick your [hotel] dates … typically the first filter set is the star rating."

Star ratings are also helpful to set traveler expectations regarding a hotel's quality and price, said Hentschel.  

"Somebody has to define what these expectations are," he said. "Then the customers' role in this all is then to use their voice and reviews to say [whether hotels] are meeting these expectations."

In a global survey of more than 23,000 Tripadvisor users, 86% said online reviews made them feel more confident to book. Users read an average of nine reviews before making a decision, saying recent reviews matter to them the most.

Though it's been argued that online reviews have replaced star ratings in the age of the internet, Hentschel said they complement each other.

"Reviews are the pot of gold behind the star rating, and every online travel agency publishes both," he said.

Stray added that savvy travelers also turn to readers' choice awards, like those published by Conde Nast Traveler or Travel + Leisure.

"There are other ways today people determine what makes a great hotel," he said. 

What do star ratings mean?

CNBC Travel reviewed numerous star ratings and found many have criteria similar to this:

And what about Dubai's Burj Al Arab — and other hotels that have been dubbed "six-star" or even "seven-star" hotels?

It's absurd, said Hentschel.

"To move up an entire star," he said, "you'd have to put a property on the moon, you'd have to put a property underwater, something that has never really been done before."

How to find a "true" five-star hotel

Hentschel said five-star hotels are "rare" and "unique in every way," with amenities such as luxury bedding, over-sized marble bathrooms constructed with granite and marble, and original artwork.  

Most have "butler service, valet parking, gourmet restaurants, lavish spas, full service health clubs and personal trainers," he said. "Heated pools, plus golf courses or tennis centers may be on [the] property."

Capella Singapore's Stray said a five-star hotel is defined by three distinct characteristics.

"If you want to be a true, high-end luxury five-star hotel, your physical product needs to be immaculate," he said. "It cannot be a cookie cutter hotel."

But that's not all. "You don't win by that," said Stray. "But if you don't have it, you're not in play."

Secondly, a true five-star hotel must employ people who understand that "everything revolves around the guest," he said.  

"It's not easy to work in a luxury hotel. It's much easier to work in a more streamlined hotel because we demand so much more," said Stray. "It's about the old concierge's kind of code, which is 'The answer is yes' … as long as it's legal and morally correct."

Lastly, a five-star hotel must reach guests on an emotional level, he said.

"There's multiple aspects that we drive in order to allow for guests to have that extra feeling that says 'Wow, that was special,'" said Stray. "The emotional connection is what people will remember." 

Soon, there may be a fourth requirement.

Five-star hotels in Dubai must have a written sustainability policy that addresses energy efficiency, water conservation and a preference for local, fair-trade and environmentally friendly goods.

"That is actually not something of the future … it's already in motion," said Stray. "And it's not necessarily because a third-party provider or rater says it, it is because our guests require it, our society requires it."

Clarification: This story has been updated to clarify that some star ratings are awarded by industry councils that work alongside government entities.