I believe there is a 10% margin of error on BG machines.
Hubbys always reads lower than mine. I just take an average if I do 2 readings and don't worry
I'd say far more than 10% with the codefree.... The one I bought was 1.0 higher than 2 other machines of my mates that read the same lower number. My research since indicates others have had issues too.
My advice is to stick to one meter only. Put the other away for emergencies. Otherwise you will get stressed and confused. You are only looking for trends and patterns, so the exact figure doesn't matter too much, and over a period of time any odd readings will even out.