TBE - If you had only played one round in your current tier, then that would be right, however if you have played more, this happens:
64, 64, 62, 62, 62, 62, 62, 62, 61, 62
The above gives an average of 62.30. Add a 61 in to it, and the average goes to 62.18
As you have played more than the above, the average score is affected even less (hence why moving up tiers is so difficult under this new system - you have to play well many times before you're ranked with the top boys)