You had me stumped... After some searching I found the following explanation thanks to William Llewellyn:
"Proviron utilizes the same 1-methylation we see present on Primobolan, another well tolerated orally active compound. Alkylation at the one position also slows metabolism of the steroid during first pass, although much less profoundly than 17 alpha alkylation. Proviron and Primobolan are resistant enough to breakdown to allow therapeutically beneficial blood levels to be achevied."
Please Scroll Down to See Forums Below 
