This is in my case only, obviously different universities will have different marking schemes with different meanings and so on. Also some people will find that they don't have to work as hard at, say, an 80, as others do.
I find that if work I hand in gets an 80, I really deserved it. As in, I put in an enormous amount of effort. Above 70 isn't too hard provided that you apply yourself and do all the assigned readings and stuff. The real difference, I think, between a 75 and an 80 seems to be more about writing quality than about quality of research. Grades above about 78 are also given for fairly creative work unless it isn't so creative but extremely well executed. If you get a 78 or 79 instead of an 80 (has happened to me twice!) it's usually just one problem with it and the assessor feels that the presence of this problem renders the essay 'unworthy' of a high distinction.
This is for Arts though, some subjects are much easier. I got an 84 for psychology in my first semester with very little trouble. All I did was go over my lecture notes and write summaries for the little topic quizzes. And my research report just followed the guidelines which were very clearly set out.
I personally haven't dreamed of getting a 90 in an arts subject this year (even though I got an 89 for my second semester of UMEP, go figure), but people do! Usually it comes as a surprise.