From what I've read, it has to do with batch variation.
From AAFCO's website:
The Association of American Feed Control Officials (AAFCO) > Nutritional Labeling
Important to note is that a single analysis of a product may not provide sufficient data to determine reliable guarantees. Guarantees are declared as either “minimums” or “maximums,” meaning that if a feed control official obtained a sample of the product and tested it in his or her official state lab, the crude protein and crude fat must be NO LESS THAN the stated percentage on the label, while the crude fiber and moisture must be NO MORE THAN the label declaration. Each batch of finished product is going to vary in composition due to variation in ingredient composition, mixing rates, and amount of water driven off during the cooking process. Unfortunately, a single analytical value does not give any indication as to the expected batch-to-batch variation. As a result, the composition of the batch that was tested may be very different from the one the feed control official analyzes.