It's good to read the FAQs - a lot of truth inside! Alas, here, the truth is - ehm - hidden in a maze. Truth is:
Your Average Score will be based on ONLY your best scores, so completing a bad round will not negatively impact your [...] Tier.".
This has to do with the non-linear aspect of average calculation. If you need more explication, just ask here - in the FAQs, the maze becomes more intensive concerning this subject.
Also, reluctantly, I have to say that some development of the game has surpassed the FAQs - they may be wrong to some extent.