Boost question

I have been trying to figure out something and need some help/advice.

This is all based on 3rd gear logs same map, same road, same day, same song on the radio;

So if i start the run in the 2.5k - 3k rpm range, a lot of taper and boost loss at 5.5krpm to 6k.
However if i start the run in 2nd gear, then shift to 3rd, (rpm falls just above 4k) i then hold the boost perfectly to 6k.
Difference between them is 2 psi, (11psi on the former and 13psi on the latter at 6k).

This question might be related to different air flow at different RPMs or the rpm the turbos are spooling, but i do remember 14 years ago i failed all classes in science (physics, biology and chemistry) so let me know your thoughts please

Thanks