Should it be "年终评比"? If your boss has to point out things like this to you, he might feel a little petty, so he says nothing. If there are multiple stuff like this, and he can give you a 4 (Meeting expectations) out of 5 (Exceeding Expectations), he might give you a 3. He was exercising his "judgment call."
As to your asking him questions, maybe it's in the way you phrase the questions or your tone. Does it show that you own the problem, or you think that he owns the problem.
Instead of asking what it is directly, present your ideas about what it might be. Show that you have given it much thoughts. And then ask him to confirm.
Demonstrate that you take ownership of what you do and eagerness to take problems off his shoulders.
