Do note that blood test meters have to be within 15% accuracy - and this is actually a fairly large amount - for example if you take the number 6, then 5.4 is 10% less than that and 6.5 is just less than 10% above that, so both are within stated accuracy levels that they have to be within if your high number was in fact 6 mmol/L.
Some meters will read lower than others but still be well within their stated range
There are some details here (at least for the UK):
https://www.diabetes.co.uk/blood-glucose-meters/blood-glucose-meter-accuracy.html