Let's break it down: Suppose the house (the site investor) is currently playing with a 90 BTC bankroll and you invest 10 BTC. You will have 10% of the new 'current bankroll'. Each bet that a player makes is played against the current bankroll. As players win and lose, your investment grows or shrinks by 10% of the overall house profit. When you withdraw your investment you get 10% of the current bankroll.
Dooglus - I don't think this wording is what you mean; at least I hope it isn't. If someone "invests" 100 BTC to take the bankroll up to 200 BTC, and I then withdraw, surely you don't mean I get 10% of 200, i.e. take 10 BTC of the new investor's money?
The wording needs cleaning up.