There are a lot of questions and odd situations that seem to indicate that he was not a communist but simply someone who was trying to convince a lot of people that he was a communist.
Then again, there were his subscriptions to communist party newspapers, and his claims to Mexican authorities (apparently backed with documents) that he was a member of the American Communist Party and also the Socialist Workers Party.
The most likely scenario is that he didn't really know where he stood politically.
And, BTW, the Russians weren't too happy to have him, and probably were happy to get rid of him. He was initially denied entrance to the USSR, they refused to make him a citizen, the KGB was distrustful of his motives, and he was only given a 1 year temporary pass to stay because he attempted to commit suicide. That temporary pass was renewed once, but he he didn't stay for the full term of the second year. The US, on the other hand, was happy to get him back - after all, here was someone who defected to the USSR, and ends up coming back the US because he was unsatisfied in the USSR, great publicity for the US.