Most likely explanation: you're seeing the stuntman (who has facial hair), rather than Mark Hamill.
It's funny sometimes how they don't make the stuntman/woman look like the original actor, but then they figure you will only see them for a split second so you won't be focusing saying "now is that really the same person or just somebody dressed like them?"
In the prequels Lucas is taking the actor's faces and digitally "pasting" them ontop of the stuntpeople's faces. On the one hand this increases realism and suspension of disbelief by making it look like the "character" is doing everything. But on the other hand the stuntpeople might be understandably upset that they have even less of a presence in the film. But one could argue that their job isn't to stand out, but to blend into the action. After all, you aren't supposed to see the technicians with their boom mikes in the background either, so the less you see of that in a fictional movie meant for entertainment, the better.