Why does he treat me better than his girlfriend at work?

I don't know what goes off in these two peoples relationship behind closed doors and it's none of my business either. I'm not even sure they want people to know they are in a relationship at work.

But there's this guy I work with and we've made good friends, whenever he sees me he comes to chat with me. He always super friendly and kind towards me. He's also a lot taller than me and will always walk at the side of me instead of in front which if you're small you'll understand how annoying that is when tall people walk in front of you. If I'm leaving a room he'll leave with me and go to wherever I'm going to help me. But when he's with his girlfriend he treats her like she's invisible and I actually feel really bad for her and as his friend I feel like I should tell him off a little for it. If she goes past him at work he doesn't even acknowledge her. If she's in a room with her they talk really quietly and awkward as if they are uncomfortable to be around each other. At first I thought they were brother and stister with the way they act around each other. If he's leaving the room with her , he walks off without her whilst she's still getting her bags which I think is really rude and I know my boyfriend wouldn't do that and he'd help me with my bags.

He's only 20 and maybe he hasn't learnt how to treat his girlfriend yet but I find it very concerning
Why does he treat me better than his girlfriend at work?
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