I see your point, yes, you would "only" need a block of 10 to be sure that you have one of each plate position and therefore that your block comes from a sheet with no font variations. But how many people collect their precancels in blocks of 10?
Here are my four singles, just for laughs:

The "usual" font variation on UT stamps is a much larger U with a wide curve at the bottom. Others are of course possible, but I don't see anything.
But of course, what my stamps show isn't really my question. My question is why they bother listing two types that virtually no one (if anyone?) has the wherewithal to distinguish between? These stamps are relatively common and cheap. Without a better explanation, it just smacks to me of someone on the "in" who has a block of 10 and wants to be able to call them something special. Which is ....... unhelpful?