There's actually data on this:
Boozer was listed as 1/4" taller without shoes during his combine measurement than Ace. Given that Ace was measured less than 2 months ago, I'm guessing his height has not changed.
As someone who's growing older, I know that losing height over time is a real thing. To me, that's the likeliest answer (if we assume that mistakes were not made in measurements, and that the other picture out there, which seems to show Ace even taller in relation to Boozer, isn't "lying" to us).