Why on earth is it a full time contract position? At least in the US that means you're cutting ~40% of your paycheck to taxes and receive vastly fewer protections than being an actual employee. You're often treated differently by staff as well... with even less job security.
It's pretty standard to bill double as a contractor to deal with the extra overhead, something tells me Jagex is not paying double.
Fixed term employees get full pay and benefits just like 'Full Time' members of staff. It is simply a temporary position.