The course is dotted with invisible ranking buoys that are used to determine the distance left to be covered.
As long as these buoys are not validated, the distance left may be wrong. Once they are validated, there may be a significant change in the remaining distance and, consequently, significant jumps in the provisional ranking.
This is one of the reasons why the provisional ranking has no strategic interest. What really matters is the analysis of your position in relation to the course, and the strategy you choose.