You guys should really add a security check for subsets and alternative MOCS inventory is the full or a subset of the original sets inventory or the check will fails and the author needs to fix their inventory or submit it as a normal MOC.
I'm not on my pc so I can only take a quick look my phone. It could be that the sub sets have errors, like other mold versions of parts. I would need my pc with my test account to go deeper into it. I'll probably can do it tomorrow morning.
I think it's has something to do with printing pattern so the machine that put the arms in do it right. so you won't end up with minifigs with pattern on the back or arms the wrong way depending on how you see it. Machines are dumb they need black patterns like that to identify heading. it much easier then learning the machine all different prints.
Well that my theory...I have no way to confirm it but it makes sense to me.
Rebrickable has a data set with all sets' numbers and the year they entered the market. I wonder if you have the dates where each set was retired (if indeed they are ever removed from the market). That is, are the sets that are offered in a particular year offered as well in all the following years? I hope someone can answer me that.