Because seeing another girl have sex even if its on the computer can be a turn on to alot of guys. Especially when you are doing it behind her back the whole suspensful act of you mastubating and her not knowing increases the level of hormones resulting in faster ejaculation. Its like when exhibitionists perform sexual acts outside or in public they strive for that rush they get when they know someone can see them at any given moment it makes them hornier.

Because your own hand knows what turns you on and what feels good a lot more than your girlfriend does.

This is not abnormal at all. When you take your dick in your hand you know exactly what to do to feel good. Your girlfriend doesnt and her pussy will feel different, so you will cum slower.

Totally normal and nothing to worry about.

because your girlfriend does not excite you as much as porn does