Let me muddy the waters further... omnivehicles!
Let's say we put just one or two big guns for one config... then what do the rest of the crew do?
OTOH, if it's a config with lots of small guns pointed in all directions, then a crew of even five might not cut it.
I actually don't think it muddies it much at all... as long as you are willing to accept the idea that the vehicle crew count may be variable based on other factors. You just have to be willing to accept that a different variant might require less or more crew members. That your crew could actually vary based on that. Slapping on one set of modules to create Variant B of some vehicle might mean you need one less gunner. Or one more.
I know that's not what the rules say but I'm starting to think the 1 per 15 tons rule is more like evaluating a car based on how many seats it has. I might be shopping for a car and see that it has 4 seats. That doesn't it's always carrying 4 people when it's on the road every single time. It just means it has capacity for 4 people.
I'm starting to perceive the 1 per 15 rule that way. It's still a good guideline but an extreme level of extraction based solely on vehicle weight and no other factors. But it might be perceived as a rule for determining maximum crew capacity and a general idea of a crew count, rather than an exact number.
If the BT writers ever got to the level of truly detailing each make/model vehicle based on other factors. Electronics, number and type and placement of weapons. At a ruleset like that, I think same-weight vehicles (and variants, including Omni variants) would have very different number of crew members and cew composition. The current rule is a... frankly oversimplified... abstraction.