When I play Viper, I get like 16-17 RR maximum per win. When I play Sage, I get like 23rr. The results of the matches and ranks of opponents are pretty similar too. Weirdly enough I usually bottom frag on Sage while on Viper I get like 2nd most frags most matches. On Omen I get +20 rr usually while also bottom fragging.

Only thing I can think of is MMR system looks at your utility usage perhaps? I hit enemies with my abilities more on Omen and Sage (flash, slow orb, etc.) than I do with Viper (no one pushes viper smoke or molly)