Does it make sense that Dragonborn A breaths fire on someone, deals X damage and thats it, but Dragonborn B breaths fire on someone, deals X damage and they become marked, i.e. distracted and suffer penalties? No, it really really doesn't. It makes no sense what so ever. But it's RAW so play it that way. Not a single thing in 4E that isn't a carryover from previous editions makes any sense at all. Just follow RAW, it's the only thing you can do, you can't read RAI out of the 4E train wreck, don't even bother trying.