Because they are?
Find me a line in any of the star wars movies or SW Rebels that calls an E-11 an E-11.
The ESB Version as i recall was known as the E-11B.
One fan nickname for the TFA version is the E-7-11
Could call it E-11C, E-11FO... take your pick.
I've heard it called the F-11-D too, which I rather like.
Still a stirling L2A3 based, the only really BIG difference is that it's mirrored.
I enjoyed reading this: http://www.whitearmor.net/forum/topic/31035-new-e-11-f11-d-blaster-episode-vii-the-force-awakens-random-thoughts-about-production/
and certaintly agree with why it was mirrored - trying to draw my E-11 is HARD. (reach down, swap hands...)
Edited by DariusAPB