Well, there are several things they could be seeing. People get banned from sites for many reasons, so I won't completely help you with this but ever heard of cookies? Any chance you didn't remove the cookie for this site before trying to register again using a proxy?
Also, because you apparently are clueless, using a transparent proxy doesn't hide your real IP Address.
I'm not clueless pal, I know transparent proxies don't hide your real ip address hence the name transparent and I've tried clearing my cookies, that was the first thing I did.