Why does he lie to his girlfriend about me?

I'm completely in love with my best male friend. he's been in a relationship for almost 5 years . I've known him for almost 2 years and have never met his girlfriend - she lives away. we're very close (to the extent that people think we are together) but I've never told him I have feelings for him. I know it's wrong for me to have these feelings for him but I've never acted upon them. just recently, I found out that he lies to his girlfriend about our friendship. we do spend a lot of time together but she doesn't know this.

why would he lie to his girlfriend? surely this makes it sound like there's something going on. could he have dealings for me?