This was an interesting nugget when I first read about the challenge system that I hadn't appreciated before. Well, to go back even further, I hadn't realized until the whole robot discussion started that the actual strike zone was if it crossed the plate anywhere in the 3D space of the plate. Growing up I'd always assumed it was as it crosses home plate - and maybe that's practically how it's going to work in most cases except for extreme backdoor pitches from the side or top.
In any case, it makes sense that they're just taking a cross-section in the middle of the plate as a sort of average/midpoint compromise, but it's still not going to be 100% perfect. Still, nearly everything will be correct.