There have been about 5 different box sizes since MP10 so far and the original mold uses (10, 11, 12, etc) have not even come out in order.

All the numbering can tell us is what shares the same mold and it hints at the characters name.

But the numbering is a quirk that Takara have been using since the MP-10 reboot and just like those black boxes the MP comes in, it would annoy me if they changed it. Through my work, home life, and collecting i really like consistency (not a big Ultra Magnus fan though).