Just to repeat, this was said to me by a partner at one firm - I have no idea what other firms do/what other partners feel. It also isn't my opinion.
As to statistics, this is from 2018, but:
https://www.timeshighereducation.com/news/universities-biggest-shares-21s-and-firsts-revealed#
If you click the table and re-order it by percentage of first class degrees awarded, you'll see that Oxford and Cambridge are down in 11th & 14th place respectively. Surrey, UEA, University College Birmingham, Greenwich, West London & Anglia Ruskin all award a higher percentage of firsts than universities like Bristol, Durham, St Andrews etc. Even if you discount music/drama conservatoires because their grading/awarding is done rather differently, there's a trend showing.
I'm certain that there is an element of snobbery and mockery present, but equally there's a grain of truth too. Then again, are some degrees more difficult than others/more difficult to get in for? Scientists would likely make the standard charge that arts students have it easy, and perhaps that is true. As Jessica said, the percentage of private school students/wealthy internationals skews things too.