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Costco, a stock we own for the Charitable Trust, reported sales numbers for the retail month of November after the closing bell Wednesday night.

As a reminder, Costco's reporting structure is unlike most public companies. In an era where quarterly reporting has become the norm, Costco stands out as one of the few that reports its sales results every month in addition to its regularly scheduled earnings reports. We value this transparency because it keeps investors informed about how the business is doing.

In the four-week period, net sales increased 15.7% year-over-year to $18.13 billion. A strong result, but the same-store sales (also known as comparable sales, or comps) is the metric we want to focus on because it provides the greatest insight into business trends.

• Total comparable sales increased 14.1% in November, missing estimates of 15%.
• Analyzing comparable sales helps us understand if the company can attract more customers and sell them more goods.

In the Costco report, you will also notice a second measurement of comparable sales. The other comp excludes the impact of the changes in gasoline prices and foreign exchange. We consider these as "core comps" because it removes the impact of items that are out of the company's control. We think core comps tell the better story.

• In the period, Costco's total core comps increased 9.2%.
• In the United States, core comps increased 9.1%, missing estimates of 11.4%, while Canada was up 7.6% and Other International increased 11.3%.

Online sales growth slowed with e-commerce core comps up 11.7%, but one thing to keep in mind is that the company continues to face difficult comparison from one year ago.

The bottom line: Costco posted another strong monthly sales result, but unfortunately, the retailer broke its winning streak of better than expected numbers. A small miss was bound to happen at some point, and we are chalking this up to nothing more than the expectations getting a bit too bullish.

With core comps still up roughly 9% for the total company in November, we see no reason to change our thesis that Costco is gaining new customers and taking market share.

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 (Jim Cramer's Charitable Trust is long COST.)