There's no rules on this per se. I do remember when there was nearly always a 10 cent difference between regular/mid/premium like clockwork regardless of the price of regular. I remember one trip to AZ where it was .799/.899/.999. But now it's not so common - especially with gas so expensive. Around here a 40 cent difference isn't unusual.
I had a similar experience about 2 years ago. Found a local Shell station with $1.99 on the sign but $1.19 on the pump. I filled up and told the family to fill up on the way home if possible. The next day I drove a different vehicle and the construction fences were already up.