<p>I do agree that resources vary across colleges, but I don't think varying quality of tutors is particularly an issue, since movement between colleges as a tutor is fairly common. As a current student I would say that Norrington table rankings are more dependent on a) cash and resources and b) the working "culture" at each college: for example, Merton (which is usually top three) has a much more aggressive?/firm attitude towards work, with penal collections and more penalties for not doing spectacularly well.
In any case it is worth bearing in mind that the actual, as opposed to statistical, differences between number of Firsts is really very small, and can be swung radically by one or two people.</p>