Most people do not have 100% identical hearing in both of their ears. One is usually better, although sometimes very slightly, than the other. Even though in biology we use terms like "bilaterally symmetric", we are far from mirror images of each half.
Still, I'm sure there errors in recording mastering, or perhaps it's a desired effect to not have vocals in the middle. Or, it's slightly different amplification itself in your system.
Who knows.... There are so many reasons for why someone might be hearing vocals slightly off center.
When I listen to live vocals, which I do all the time since my wife is a singer, I can always pin point exactly where she is with my eyes close including when she is perfectly centered infront of me. When I listen to headphones though a lot of music is panned to the left.