What's happened is that he's probably given an initial mark, then after marking all of the assignments, he's adjusted some marks slightly. I do much the same in my own marking of assignments. I'll start by marking everything, then I'll go back and reflect on whether I think some of my earlier marks are fully representative of the standard of that piece of work. For example, I might mark the first assignment a low 2:1, but when I see what else I awarded a low 2:1 to, I'll then adjust that early mark higher if it's of a higher standard, or equally lower it if it's not as good.