I can tell you that I had been quite an active Hubber for some time, but then left for a couple of months on personal business. Leading up to my two-month departure, my commenter level was a solid 7 and I thought I would be reaching level 8 with the very next key stroke. :0)
That moment never came. Upon returning I was still a level 7, but one day (out of the blue)...I dropped to a level 6. The next day I was back to level 7. The day after that I returned to level 6 and I have been stuck there for the past two months or so, even though I have been quite active.
I notice that some Hubbers have achieved level 8 commenter status, but I usually only see them render very short answers to questions and their rebuttles to Hub comments are usually short and sweet as well.
I do not know what the criteria is for moving up or down the scale, but in the grand scheme of things it probably matters little. I think the true measure of a commenter's status is achieved through the quality of their comments...not the quantity of their answers.