I think this is all a bit of a cyclic phenomena. I never expect a good days readings if I was naughty the day before. There seems to be a lag in my case where I need to concentrate on the diet for some days before things slowly improve. One days naughty seems to result in one days misery. A couple of years ago I experimented with tying one on and eating buns to see what would happen. What happened was I got high readings afterwards as though there was a baseline to the graph which had got higher.
Behaving for some days resulted in an improvement of between 0.1 and 0.5mmol each day until I, once again, got down to my good baseline. It seems you can't have a one day splurge and expect good results. You have to have a good day before the next day is even better. About five days was what it needed in my case.