It might also mean they haven't come up with a name and wish to see where the design is going before fully branding it. In any event, it's 5E to me both because it makes sense and because it's faster to type than Iteration.
I think it's mainly this. 'Iteration' and 'D&D Next' are stopgap names until they finally decide on what they want to call it.
If they started referring to it as a fifth edition or 5E from the very beginning, everyone would basically hitch themselves up to that name permanently and they'd never be able to eventually call it something else even if they wanted to. At least now... while some people will call it 5E regardless of what the eventual name is... many other folks will be able to let the 'iteration' and 'D&DN' names go and latch onto whatever the final decision is on the name.
That being said... I do think that there is a bit of marketing in there as well... in that saying '5E' implies that it's a new version of the game, whereas they're trying to get across the idea that it's actually an adaptation/evolution of all the previous games. Yeah, that might be purely a semantic argument... but that's marketing for you.
I still intend on getting into my 'pre-owned vehicle' come release day and drive to the store to take a look at it.
