Ya, a few of my hubs were above 80 yesterday and have dropped to 75 overnight.
But I'm still very new here so I'm not that concerned about it really.
Also, your hubscore is a direct indicator of traffic to your hub...so obviously it'll fluctuate with traffic.
When your hubs are fresh and are being read here, your hubscore will rise. Once they become old, it all depends upon referrals and organic traffic.
So, Try promoting your hubs on other platforms too coz only that's going to give you true organic traffic...something that Google loves.