As you can see, they are graduated in size; I don't know if they're a set or if they've been put together from sets of same-sized dice (it wasn't the only such set for sale), but I suspect the former because they would probably sell better if they were the latter. I also suspect that there were six dice in the original set, with the largest ones sold separately as they have a tax stamp on them and the rest don't.
Now although these dice were probably made to comprise a set, they're not uniform. Three (1, 3 and 5) are right handed but two (2 and 4) are left-handed. Three of the threes (3, 4 and 5) slant like a slash but two (1 and 2) slant like a backslash. Of the numbers you can't see in the picture, if you roll from the one to the two to the six, four of the dice (1, 2, 3 and 5) have the pips on the six run left-to-right, but one (4) has them run front-to-back. Only two of the dice (3 and 5) have their faces and their pips aligned identically.
Now clearly there are some rules that the die-manufacturer has followed. All opposite faces sum to seven, for example. They haven't been consistent elsewhere, though. Why is that?
I'd like to think it's because dice are meant to be used for generating randomness in play, therefore adding some randomness to their manufacture is just a reflection either of that randomness or that playfulness. Then again, it could just be that the people chopping up ox bones into cubes didn't really care.