Probably two possibilities 1) when system was drained, it also flushed out some sludge, so system is no operating more efficiently and hence rads are now getting hotter or 2) you had some air in system which was removed when he drained and refilled system again making system more efficient. Nothing to worry about only plus points as you should now use less gas. As an aside do you know if your plumber put some rust inhibitor in system when he refilled - should be on invoice, if not needs to be done to prevent future problems.