I had that problem when I would store the number in the address book and not use the area code. The BB would see it as a totally different number than the one that shows up on the caller id with the area code and doesn't associate it with the one in the address book. It's just like the difference between a 5 digit and 10 digit number they are not the same.

Actually, I have had this problem, and still do. Not sure what causes it. Sometimes when the phone is in it's case and rings, I take it out and it just shows a number. Then, sometimes if I just have it sitting on my desk and it rings, it shows the caller ID. I've played around with it so many times that I just gave up. Maybe just a simple bug... The weirdest thing is that when it doesn't show the name when it rings, and you look back at the call history, it shows the name there. Who knows...

Maybe you should check whether you double saved that number ?