Sounds right to me from what you post about your workout habits and overall health. If I were to drop down and do pushups right now I'd probably only be able to knock off 35-40 but if I do them routinely for the next week I'd get up to about 60. I know this to be fact because I've taken breaks and got back to form quickly. I think its all about muscle memory/toning. Since you didn't do pull ups regularly before, the muscles weren't in the habit of being put to that specific task, but after they were put in a pattern, they can produce much better.
