The style of the lettering on the -EN coins looks different to me than the two SEN coins, IMO. Look at the E in PF EN AVG in the coins on the right - in several of them, it is oversize, compare the size of the E in SEN on the left. And the P's are different also. I bet it's just one single engraver who cut several dies and kept making the same mistake, while the other engravers spelled the dies correctly.
Coins of Maximianus as Senior Augustus are not exactly common from Alexandria mint, there can't have been all that many dies - could be that most of them were created by the one engraver?