Maybe the springer, chrony and BBs are fine...but your other builds are shit? (JUST KIDDING.......)
When you get a different reading with a given rifle...do you still get the same variance (+/-fps)?
I definitely get different reading depending on how close the muzzle is to the front sensor or how high/low it is.
Trick 1: tie a pipe cleaner or bit of wire to your chrony and have it come out towards you and up to where you want your muzzle to be...then just get your muzzle to touch it and shoot
Trick 2: set it up in a shooting box (or on a table) and rest the rifle on the workbench. just make sure you have a good bb trap