You get a lot of additional scoring for action time (average time per mech you were alive), especially in control point clash.

So if someone just sits around doing a little bit and happens to live a long time they end up with a lot of score.

I think it's a bit dumb but oh well.

I'm assuming you're talking about how X player gets an MVP here or top spot but it's not super clear

im asking how he got 700 xp more than me

It's CPC, so captures hold much value. Same way for 5vs5, it's assists (per my findings).