Imagine if you see two universities that you like equally!
Uni 1 is ranked 5 on the league tables for an overall score but for the subject you want to do, it's ranked 50.
Uni 2: for the subject you want to do it's ranked 5, but its overall score is 50.
Which university would you choose and why?
Another question, would you choose a red-brick Russell Group university that's ranked 30 in both your subject area and in general, over a non-russell group university that's ranked 20 - in both your subject area and in general?