Because league tables are meaningless.
I read an amusing article claiming all the most intelligent prospective students go for uni's higher in the rankings!
To be honest if you are dumb enough to think that some very arbitrary and outdated rankings give any great indication of what a university is like then i would suggest you aren't as clever as you think...........
Firstly you have to remember that some of the data used for Manchester is well out of date. With the Merger in 2004 everything's changed but statistics have yet to change. The next RAE is next year and i'm pretty sure with all the new investment Manchester will come very high up the pile.
Secondly, Manchester isn't particularly fairing well in the newer rankings because of the recent addition of student surveys. Manchester faired badly in the last one, no doubt because the survey was probably only completed by a couple of anarchist students pissed of at evil organisations like uni's
I hate student surveys, if you know anything about students you'll realise the people that respond to them are usually not that representative of the student body (pretty much like the student union
). With Manchester's history of a very active SU then the results are unsurprising.
I also don't like the student spend factor in rankings. City universities don't need to provide so many facilities for students unlike campuses in the middle of nowhere *cough*Warwick*cough* so i always am dubious about those.
Manchester usually does well internationally due to it's historical scientific leanings, for example some rankings emphasize Nobel Prizes which Manchester has quite a few of compared to universities of a similar standard. Manchester (and the old UMIST) have considerable international reputation.
All in all, the league tables are meaningless. The methodology of most is entirely subjective to whether you think that makes a good uni or not. UoM is a very strong uni and with the current massive investment is no doubt going to get better.