You are one of the lucky ones! I'm always a bit jealous since I sink much easier than remain buoyant.
Based on observation, I haven't had a female sinker (at lease not yet) and all float quite easily, even after emptying lungs. It's largely understood or accepted that body fat is more buoyant than lean muscle, women are typically 10-15% higher in body fat than men providing that added buoyancy.
That said I've had some very thick, not so lean guys, head right to the bottom with lugs full, as well as some very lean, male (5% or less bf) runners bob effortlessly on the surface, including legs. Body builders (guys) on the other hand, generally sink fairly quickly. It seems the frame, size of bone and its density is also a big factor too. But this is only based on my observations, not necessarily science or fact. Size of lungs I'm sure is a factor as well.
Coach Suzanne, "Dr A" probably has a much better, more refined explanation if she sees this thread.