The D IS counted as worth more, so it's paying for those extra points of damage; specifically +5 PV for more Short, +2 PV for more Medium, and -1 PV for less Long. So, uh, +7 PV?
Remember that Short range is only 6", and that the Turkina is slow; in all likelihood it may get to use that extra Short range once or twice a game, whereas Medium range is where the majority of a game's shooting takes place. It's also not terribly difficult to stay out of Short range of such a slow unit, so the extra PV would go to waste if they knew you had a Turkina-D.
It's not like, say, the
Fire Moth H where that massive Short 5 damage also comes with a blistering 26" movement, after all.
It's the same reason that Armor is worth more than Structure: Medium range damage gives more value than Short range damage, so it has to be worth more.
As far as whether it's worth the upgrade... The B costs 12 points per Skill upgrade, the D costs 13, so not a huge difference; but if I was going to reach for another Turkina for massive damage it'd be the E. I think 10 Medium might be a record-breaking amount of damage for a BM unit.
Frankly, if I had a Turkina, I'd probably have the E on one side and the C on another. The C only costs 11 for Skill upgrading, and a 6/5/3 with OV2 is a reasonable damage statline. The Prime/A are also frightening snipers; very few units at all deal over 3 damage at Long range.