That happens when using the same finger! When I first started finger pricking, I ended up with 4 glucometers, one from an initial GPs visit, then another a day later from a hospital admission, then one sent for 'free' from a diabetes site and I'd already bought one when my symptoms first started. Much experimenting showed that there were differences (of up to 2mmol) between all four when used simultaneously, between fingers when using the same one, on the same finger when using the same one or all 4, and on and on. I am sure that it's because our bodies are very complex and each drop of blood is coming to the surface via a different route but also because the glucometers et al, have not yet been designed to be accurate to more than around a 1.5 mmol difference I believe. In the end I chose one to use (the cheap one I get on prescription) and stuck with that. Now I have the libre mind I get the libre strips to go with it, they all seem similar though.