Why is eye contact wierd with actors/actresses?

I know, it's hard to explain. I'm watching Final Destenation, and the scene where Ali Larter is explaining her sculpture to Alex, and in the shots where she is looking at Alex, her eyes are moving left to right instead of looking in one direction. I've noticed this in other movies/TV shows, and always wondered. Idk, I know it's stupid. Lol.