There might not be good reason, but I don't "trust" calculations...
from a bunch of tests that I did (a while ago)...there was less of a drop going from 0.20's to 0.25's...and more of a drop in FPS from 0.20's to 0.28/0.30's than what you calculated. * chronies can be off a bit too...it's not like they are super high precision pieces of equipment.
Cutting through all the "I calc this...but I shot that"...if people just chrony with 0.20's there's no ifs ands or buts.
and as long as the host decides to a method/number and sticks with it...that's what it is, no matter how right or wrong it is.