Yeah, the gap between A and A+ in work quality is quite small, but as results go up it's exponentially harder to get certain scores. What I mean by this is that few things differentiate A from A+, but in order to get the extra few marks that bump your A to an A+ is hard to get. I hope that's understandable.
In any case, remember that everything in VCE is graded on a curve, which means that you have to beat a very high percentage of people in order to get the higher marks. An A itself is quite an achievement, even though it's very frustrating to not get the A+.
If it helps, it's usually much easier to score higher on the end of year exam because by that stage, many more students have 'given up' and you know more about what's expected of you.