Show me where you saw that (in dungeon i mean and solo).
On a normal mob it's probably normal as they can deal pretty decent damage solo, because their higher skill isn't depending on placement (front, back, rear) but just a casting time that can be interrupt.

Also, i saw a lot of people having a full ilvl310 with Materia V melded, probably your sam you saw was full Melded VI. (the gap dmg on normal mob become more potent).