OK - maybe the difficulty has an impact - but the usual question is about how much the average moved, compared to just before the tier advancement, when the Average hardly moves at all! This has nothing to do with the difficulty of the new tier.
The Average resets every time you advance to a new tier. After your 1st round at your new tier - your average is simply that score. Your 1st score was 84. So your Average was 84.
You have now played 3 rounds. 84, 84, 74.
Your average is therefore (84+84+74) / 3 = 80.67