I can't be arsed to find statistics, but having had this discussion many times before (including with university staff) I am fairly confident stats would back up the following assertion:
Arts and social science degree grades heavily cluster around a 2:1, whilst math and science degree grades have a considerably more even spread across the range 1st to 3rd.
With an arts degree getting a 1st requires seriously hard work, but as long as you can write coherently and reference your essays then getting a 2:1 is relatively easy. I probably averaged less than 20 hours work a week to get a 2:1, and this was not at all uncommon. To get a 1st you have to work more like twice that, and so generally people just don't bother - I seem to remember only 2 or 3 people in my year got a 1st. You have 2 lectures a week, if you even bother going to those, so getting a 1st requires dozens of hours a week spent studying books in the library. Outside of those with serious ambitions to get a top-flight career or go into academia, there is no motivation to do so - you're better off either getting relevant extra-curricular experience, or just enjoying yourself.
In the sciences there is no shortage of people with a natural aptitude who find large chunks of their course easy, and given the courses are lecture intensive, and people are under close supervision to hand in problem sheets and complete experiments, those who do have the aptitude are generally pushed to get a 1st. Then there are also plenty of people who get to university and find advanced level maths or physics is beyond them, or otherwise just really dislike their subject and don't make the effort to keep up with the workload, and struggle to achieve a 2:2 or a 3rd.