UCL have stepped up their maths dept, asking for very high A-level grades w/ STEP. Durham have always had a solid maths course, Bristol is amazing dont get me wrong but not as good as UCL/durham.
Offers are a function of competitiveness for places and the willingness of a department/university to resource an admissions exercise that is something beyond bean-counting. Imperial, for example, one year had a requirement of A grades in all modules for maths and further maths, with no resits permitted.
In what sense has the UCL maths department actually stepped up or changed its course - it may well have, but are you just going on the increased offer or have you compared any details of the courses? And then you have just reiterated your opinion of Bristol without any further evidence.
If you look at the range of indicators that Bill Lionheart has on his webpage (which I admit is a little dated now) there's quite a few reasons there why Bristol should be considered much better than you do.