It is indeed as you say - gameplay decision. The idea is to represent effective range on the tabletop
Especially hull mounted MGs are deliberately scaled down in range to show the effect of the limited field of fire and sight - that is also the reason why the same MG has a longer range when pintle mounted.
If you like longer ranges your proposed house rule sounds good to me!
BTW - the topic you raise is also the reason, why I like to play ViDe in 15mm, the ranges look more "real"...