So while I was registering for the US Class Championships this year, I entered into the Class C division because my rating was 1434P and the Class C division showed ratings from 1400 to 1599. But after the tournament, I noticed that there were three players, going into the tournament, who had a rating higher than 1599 (see note below).
Why could these three players enter a class division from 1400-1599 when they had a rating higher than 1599?
Note: the three players had ratings of 1639, 1690 & 1702, so a couple of players were around 100 points higher than the upper limit of the Class C.
It’s because they use published ratings. There is a cut off on the first Friday of each month, and the ratings as of that day are official for the following month (for example, the ratings as of the cut off on 7 October are official for all of November). This is to facilitate planning. This way you’ll have at least three weeks to know which sections you can enter in a tournament, possibly many more. If you check the rating supplement tab on those players’ MSA pages, I bet that’s what you’ll find.
You are looking at the latest (unofficial) rating just before the event. Unless advertised otherwise, tournament directors and organizers normally use the latest (official) rating supplement. For this particular event starting on September 30th, we used the September Rating Supplement, as advertised. Using that Supplement, all players were under the Class limit. The September Supplement ratings for those you mentioned were 1590, 1576, and 1543. It is not unusual for players to have an up-to-date (unofficial) rating that exceeds the Class limit, not just in this tournament but in others as well.