Some of my hubPages have reached 99 ranked, and dropped to 46 in the same. For example this hubPages:
It reached 99 rank several times, despite this was not one of my best one! I have even better HubPages than this.
As pointed out by other answers, the rank changes - it goes up and down. But if you noticed, in your account (statistics page), the ranks average will hover around 75. I have a feeling that HubPages, they have set this rank as an average. If some hubpages reach higher rank, the other pages ranked down to make my mean rank to come around at 75. Currently my mean ranking is 74. Sometimes it goes up to 76. I have not seen the avg. rank to cross over 80 or 90 to me. About others, I have no idea!