Their increased bone density probably accounts for an increased degree of body density (i.e., they are heavier than they look).
The issue of hyperplasia in humans is still questionable. Regardless, I don't see how that would affect density per se.
It may have to do with greater recruitment of Type II fibers and more contractile proteins per fiber as well as increased bone density as a function of heavy mechanical loading with PLing.
Overall, heavy lifting is essential for size and density. My guess is that the BB routine used simply didn't recruit enough Type II fibers. Too many bodybuilders piss around with relatively light weights, high reps and foo-foo movements like lunges and other isolation movements. I think you'll find that most of the big guys and girls employ power/strength sets in their off-season routines to build size.
What AAS are used may play a role as well in those that use them. My guess is that the choice of AAS differs between BBs and PLs. None-the-less, training is still the bottom line. I see chicks in the gym that I've heard are juicing but lift light and could probably achieve the same without drugs if they just lifted heavy.
Even when I train my clients we mix in heavy lower rep sets (3 - 5), pauses, lock outs and/or speed sets for general fitness routines.
Most of my female clients use heavier weight. I have one that is 7 1/2 months pregnant and flat DB presses 35 lbs for 10 - 12 reps. Most veteran females are in the range of 50 - 65 lbs DBs for 8 - 10. Never juiced in their lives, don't even take supplements.
One of my female clients (former competitive PL/BB) can flat DB press 85 x 8 and 100 x 5.
In fact, most of the women I train look lighter than they weigh, even the housewife types.
It's all how you train. Regardless of what you're goals are, a mix of training is important unless its figure, then it has to be foo-foo at the NPC level or you'll be marked down. Of course if you're pro figure, better start training like a BB.
W6