You could work on the theory that Mould change = new number with each of those, but then you fall into issues with MP-12G and MP-13B, especially since you get Ratbat there as well....
Then I had a brainwave...
MP-12G, MP-13B, MP-21G, MP-05G are all the SAME CHARACTER as the original, just the Gen 2 version, or electrum coated, or back from the dead.
Also explains why MP-11A and MP-11SW kept Starsceams number, as they had no mould changes (I know, I should open mine and check... might be a different smirk or something) And I think MP-10 EVA is a straight repaint too (might check when i get home from work.... good excuse to get him off the shelf)
And of course if you want an exemption to the rule.... MP-10 A and B had their should leveled of the Autobot insignia, so the Ape/Decepticon tampo could go on.