Here is what the manufacturer replied:
By law, a crib mattress is required to be no smaller than 27-1/4” x 51-5/8” when measured from binding edge to binding edge. Depending on the manufacturer and style of the crib, the inside can be between 27-3/8” and 28-5/8”wide, and 51-¾” and 53” long, so there can be some space between the mattress and the inside of the crib.
We are required by law to manufacture according to specific standards. Some mattresses will fit more snug than others depending upon the density. Dense mattresses are not able to compress as easily and would serve to fill the space between the crib.
Furthermore, all interior crib dimensions and mattresses sizes are strictly regulated by the CPSC. For such concerns parenting websites and blogs recommend the two finger test. The space between the mattress and side panel should not exceed two finger widths.