Correct. PT lumber is never dimensional. Your 2x4x8 is likely 96 3/8" inches long. Or 96 3/4. Or even 97. and sometimes 96 1/16th. The lumber is cut before treating and drying, and depending upon the composition (graining) it may shrink more, or less, than a similar board, though you see much less variation in the 2 and 4 (which are neither 2, nor 4) dimensions, because they started either 24 or 48x smaller than the length of your board.
Trim to length. Chances are, your concrete floor isn't perfectly level, and neither are your ceiling joists.
If you happen to also be using whitewood studs (generally 92 5/8") those are cut to length post drying, and you pay for that in the lumber costs, since it saves time framing on site.
Regarding short (pony) walls, the unsupported corner end WILL be floppy. There is no practical way to prevent it with a floating corner. Either extend the corner to the ceiling (think of a post at the end of a short wall, as sometimes seen in kitchens with a "pass through" or a converted breakfast bar) and tie in there across your joists, or turn your wall 90 degrees at the end and create a short wall there, as you often see at the base of a stair railing.