Indeed, I don't think it accounts for much in the way of depth or new signings, because it's about the teams performance. If I recall, it uses previous seasons data as well as current - though of course you'd imagine it yo be less accurate at the start. Also will be a lot of fuzziness for...
It's based on the 'expected goals rating' that the author uses for his modelling. He explains it on his site but in brief, it is a combination of the quantity, type and quality of shots a team tends to take, and how often those types of shots tends to go in, in general. So it's prioritising...