xG is what you would expect a player to score based on the chances he has had. They just look at statistical data from similar positions in past goals/chances and use probably a lot of different fairly subjective statistical parameteters to do that. I think distance/angle to goal, speed/quality of the ball from the pass, defenders/goalkeeper positioning for blocks/saves/tackles and things like that.
Different xG sources can vary a lot too. For a pen they have 75% chance of a goal which is probably the easiest stat to look at.
Vardy though has scored 85% historically and 100% this season so he will probably beat that. Rashford and Pogba for us maybe not given the record they have.
I think top strikers normally do better than xG in many areas at least when in form. Since they have different qualities someone like Ings might be worse than average in the air compared to say Auba or Richarlison who are good there. De Bruyne as an example have pretty low xG because he scores lots of hard long distance shoots that few others can manage equally well.
Messi on freekicks as an example performs way above xG normally since he is the best in the world at that and they compare him to other players for xG. Messi in general apart from headers is normally way better than xG says he should be and that is why he is the best.
