Thing is McFarlane has a line that's just unambiguous BTAS and BTAS-related designs at seven inches. No reinterpretations, just as close as they can get to those designs.
The main DC Multiverse line, on the other hand, tries to go for that reinterpretation a bit, and with Batman Beyond it's taking a design that's almost exclusively existed in a BTAS style and tying to make it work in a broader style.
It's kind of like when Hasbro tries to do Animated in Generations and tries to marry those stylized designs with a more catch-all aesthetic.
Also, technically, McFarlane's main DC Multiverse Batman Beyond figures reference comic retellings of Batman Beyond DC's done the last couple of years on the box. Those comics, AFAICT, have a more general "comic book" art style.
I haven't read them, but they may be going in that direction with the general look.
