In IRL races, the lap counter displays the number of whole laps remaining at the finish line (blah blah lapped riders blah blah). On Zwift, it seems to display ceil(distance-remaining/lap-length), ie it changes to the number an IRL counter would show only when you cross the finish line. In reality there is a small delay and the counter is pretty soon after the finish line, meaning that the time you have to see the correct number is less than a second. Keeping the number the same (again, the number of whole laps remaining at the finish line) for the whole time the counter is visible would make more sense.
In addition to race participants, this seems to occasionally confuse even race commentators as well.