It is fairly common across the board. As you’ve already said, a lot of institutions require special reports to be written by the examiner if they award higher than 80% or lower than 30%. The sad reality is that many university lecturers don’t really have time for this with the mountains of assessments to mark, the lectures to plan, the research to do etc etc. And given everything above 70 is a first anyway and you can’t get higher than that, it’s understandable to a degree why they do it.
Also while something you have might be a model answer for a first, that doesn’t mean it’s worth 100%. Many lecturers will require something scoring in the 80s to be beyond what is expected of an undergraduate. Basically, 80 is often reserved for the “truly exceptional” which holds a very different meaning at university than it did at school.