I come back to a point I made elsewhere on this topic though. Diabetes is not a condition of exactitudes. So one meter reads differently from another? So what? As a T1, you had to use urine and test tubes and then visual colours to guesstimate what was going on. Now you get an electrochemical reading that, due to reading to 0.1 of a mmol, people believe to be "accurate".
Given that timing, absorption rate, test locations and measurements are all variable and not at all "accurate" then the numbers on blood glucose meters can only ever be a guide. The best you can hope for is consistency. Getting hung up on the accuracy of the numbers or even the accuracy of the numbers isn't going to dramatically change your life. If you target 5mmol/l on a blood glucose meter, then, based on the current and future ISO requirements, you will be measuring within the range you want. with it is 4, 5 or 6 really doesn't matter a vast amount. For a type 1, what's more important at that point is the direction of travel.