i suspect it is the shbg decrease and what masteron does NOT do more than what it does. It does not have any estrogen conversion at all. Maybe in your case the added androgenic effect combined with it not converting (like test, dbol, abombs do) is what is giving you the libido increase. Proviron does that to me big time, more than masteron for some reason or maybe i notice it more acutely. Either way, both drugs are fucking great overall, I always use one at even a low dosage because I just love the effect.
Yes like esfitness said above proviron is another very high androgenic product and will increase your sex drive. alot of people like stacking masteron with proviron.