I don't know how Bristol conduct their assessments but possibly one reason why Nottingham award slightly fewer good grades is because all of your marks come from end of year exams; for example, I have 3 exams this month, each of which is 3 hours and each of which represents 100% of that module.
This method of examination is really tough because you don't get the chance to build up a collection of marks throughout the year, through coursework or group presentations etc, as a comfort cushion in case you do badly in exams. It means that when the exam season comes around, ie now, you have to revise your ass off and ensure you know in detail almost everything you've covered all year, which means serious work. I'm sure this is a level of work which some students just can't live with, therefore they don't do it and don't get a 1st or 2:1.
It's also worth pointing out that one of the reasons why Nottingham's law school is well regarded by employers is because it doesn't mark first years generously. First years are not given a concession because they are first years, they are marked the same as second and third years.