from what I have read elsewhere in the forums, you need to reach a minimum number of ranked rounds before a bad round does not raise your average.
I believe for the amateur tier you need to play at least 10 ranked rounds before your best rounds are kept and a bad round is ignored.
Here is great description pasted from another member (he's talking about the legend tier but he gives a good description of what happens with 10 ranked rounds):
ScottHope:
Hope I won't step on any previous posters toes by adding this but I'm only a simpleton and some of the stuff in this thread is very complicated.
Your average score as a Legend is made up of your best 500 ranked round scores. When you play a round with a score that is better (lower) than one of the scores in that list of 500, it replaces it.
The amount your average score drops by is dependent on the worst score in the group of 500 that gets replaced.
For example, if your average score was made up of just 10 scores:
36, 29, 40, 38, 36, 39, 41, 37, 35, 30. (Total 361)
Your average score would be 36.1 (Total 361 ÷ 10). If you then scored a 35, it would replace the worst score in that list of 10. So it would replace the 41, therefore the new list would be:
36, 29, 40, 38, 36, 39, 35, 37, 35, 30. (Total 355)
and your new average would be 35.5. (Total 355 ÷ 10)
Hope that makes some sense.