I was ALWAYS told that there's a "blurry" side to plexi, and that's the side that is supposed to go - damn if I can remember, it's been a long time since I was in sales and had to change the inserts in the plexi-covered lightboxes we had.
That "blurriness" is because it's done on the wrong side.
Naturally they do this WAY more than I do, but you even noticed it yourself when you said "some look better than others", and I'd bet it's not a big concern to them one way or the other, so you get what they have.
Is there any way to do a hi-res scan of the backside and perhaps go that route? The screenprinting on the back isn't blurry just from the front due to the plexi, right? I know that's not the "right" way to do it, but that sucks that it's blurry when it clearly isn't that way on every machine