I did a few years ago for a brief spell, I'm sure an umbrella company is the finance department (wages clerk) for agencies,
iirc they give you options on how you would liked to be payed, either on a PAYE basis or you can settle your own Tax/NI
when doing your Tax returns, this would probably be the answer to why your figures vary so much, also I'm pretty sure
they all charge a fee for processing your wages as well some a lot more than others..