I don’t believe that Proviron is that low but yes without a doubt any oral has a lower bio because it has to pass through your liver most are methylated with a carbon atom to help go through or they wouldn’t make it and your body would push it out.
This is also what makes certain orals so toxic to your liver.
Certain orals like var, adrol ,dbol have recently been popular as injectables because of the demand for more of the product to enter the body without your body destroying it.
I believe that Proviron is actually much higher but I definitely could be wrong.