Help please

I'm still a bit of a novice on tournament outright bets but can anyone explain to me why my current book on the World Grand Prix Winner should be showing approx a £25 difference on the overall outcomes using Betfair's 'new' site to Betfair's 'old' site for any result . Both sites are set the same showing figures after commission etc . I may be missing something blatantly obvious but it's just baffling me as to why . Thanks in advance .