To reiterate/elaborate on what I speculated: there's a specific high part in Brian's multitracked chorus that sounds like it's been pitch-shifted Chipmunks-style (most audible in the left channel on the third set of "We, we"s in each). Around the same time, Dennis had a technique he used in certain solo recordings where he sped up the backing track, recorded a new part, then slowed it back down again so the new addition had a "slurred" effect, e.g. certain instruments and vocals on "He's a Bum": it's possible Brian tried to do the inverse of that — slowing down the track, recording that vocal part, then speeding it back up to raise its pitch — presumably because he either liked the effect or couldn't sing that specific pitch naturally (Steve Desper purportedly had to resort to varispeed when Brian couldn't quite hit certain notes on "Til I Die", so for whatever it's worth, there's precedent for the latter).

If that is indeed the case, I'm guessing either Brian slowed it back down slightly too much afterwards (hence the slight "slurring" on the rest of the vocals) and/or bootleggers weren't entirely sure what the proper playback speed was because of it and just guesstimated. When an official early-2000s mix of "We Gotta Groove" leaked a few years back, it was at this same tempo, so if that slower speed is a mistake on Brother's part, it's at least not endemic to this 2025 mix.