When looking at Pokryshkin's kill record, the majority of his kills on fighters were on the 109, could either be there were more 109s? Or just easier to kill them?
There were 33,000 Bf.109's produced compared to 20,000 Fw 190's; and the Western Front had a higher priority for Fw.190's (the Bf.109's armament being largely inadequate for taking on heavy bombers).
There were only two Geschwaders operating Fw.190's in Russia, JG51 and JG54. JG51 converted back to the Bf.109 in 1943 IIRC.