If you get a rise in BG levels it is either because you have eaten something or because your metabolism has released some, possibly by breaking down stored fat.
As has been pointed out, peoples' metabolisms are different and they produce different amounts of several enzymes which work on different foods eaten. Taking metformin too complicates the picture. That's why either fasting blood plasma or HBA1c are tests prefered by physicians, because they have fewer complicating factors. It is frustrating that you can't get to see a clear picture of what is going on but it's not your fault and nor does it have to indicate loss of control. It's to do with the complex interaction between a number of processes. Even if you eat nothing and go to bed and do nothing, your BG can still go up.