Either last weekend or two weekends ago I posted every stat that goes into the calculation. I'll try to find it again, but it's basically a stat that takes into account where a shot is taken, how it is taken (cross vs pull back, where the pass is coming from, header vs strong foot) and a bunch of other stuff (counter vs non) and weighs that vs other similar situations and gives a number on how likely that shot would result in a goal.
So, a shot that is from a counter taken from 6' out might be worth .6 "expected goals" or w/e.
That chart above is players they have at least 600 shots worth of data of over the last number of years and who scores more goals than expected given the shots they take. Obviously in such a model some players (the good ones) will out perform it, while the worse ones underperform.
Edit: The data for the two charts is for between 09-14.
I'm very curious where Giroud falls under here. He is very, very bad at scoring "big chances" but pretty good at scoring tougher ones.
mc of A just posted the bottom 20....It's pretty sick that City can still score 100 goals with 2 of their starters being some of the biggest underperformers in xG.