I mean, the rankings are still dictated by how many people move up past you from the day. One could make estimates at a current rank, but estimates are estimates, which can vary per day. A play that you make and would award you PP can also mitigate the rank decay that happens per day.
There's also sometimes when the ranking number gets a bit sleepy, such as your rank reverting back to the start of the day, and then going to your current updated one. When you made that top play and gained 1500 ranks, that's when it finally applied too.