As people born on February 29th can’t celebrate their birthday on the correct date every year, they are most likely to celebrate it on neighboring days.
Assuming equal amount of people was born each other day, this extra quarter adds to those actually born on February 28th/March 1st, making those days most likely for someone to host a celebration.
But that’s not the case and February tends to be one of the months with a lower birth rate.
Fair!