DISCLAIMERB: I might not use the correct terms, but, this has happened to me also.
I had this issue with one of my laptops - turned out that it was just not "pixel perfect", only I didn't know that until I got a new laptop and everything (all of the borders and such) were perfect. I don't think that is your problem, though. It's my understanding that WoW's parent company has fixed that particular problem. But, I could always be wrong.
In my defense I must tell you that I'm 67 years of age and live in an assisted living facility, and I haven't played WoW in four and a half years and I don't believe I ever will again.