You might want to count the number of skill points you have used.
Is it possible that you somehow applied the skill point you received when you reached level 20 to a different skill on the doll? I have accidentally applied skill pts to a skill just by hovering my mouse over it, trying to read what the skill does. I've also accidentally applied more skill pts to a skill than I meant to, when there was a delay after clicking on a skill, so I clicked again and ended up putting 2 skill pts there when I had only wanted to put one.
Having said that, my test character has not been receiving skill points when reaching the next level on her staff (twice now). Not sure if it would happen on another weapon or not, I haven,t leveled any others on her recently. I need to send in a bug report, but haven't done it yet. I'm sure its my graphics card that's the problem.
Edited to say sorry for saying basically the same thing Grizzly stated above. Before I started posting (and then became distracted), he hadn't yet replied.