As somebody said, when they used real Sterling as base for E-11. Logicaly they put it in holster on left, because ammo magazine, hengstler etc. They didn´t think about rebuild this weapon mirrored for right side holster. Too much work and money and almost no one will notice that.
After years we can think, that Left handed stormtroopers are superior.
For EIB ANH you need it wear on left, even if you are right handed. I don´t think they will change standard, because you are right handed or you make mirrored E-11 that can be worn on right.
Funny is ROTJ in scene in front of Endor Bunker, many of them don´t wear holsters )