I took Forbidden Material: Silver so I can tell it apart from Elven and Chain... Does that have a factor as well for odds of getting silver?
I've played many, many games collecting armor like you seem to be doing but I've never noticed the odds of finding silver change, forbidden or not.
Silver shields and helmets are relatively common compared to other silver armor. I have played numerous games in which I've found all 4 shields and the 6 different types of helmets. Of the other 15 different pieces of silver armor the most I've ever found in one game is 4. (Orcish helmets are also very rare).