I doubt the control solution is interchangeable between meters.
The Codefree is calibrated to convert the whole blood (from finger pricks) to a plasma reading (as though it had been taken from your arm). Check the manual for the Contour and see if it is the same. It will be buried somewhere in the technical specifications.
Plasma tests are approximately 12% higher than whole blood tests. Could that explain some of the differences?
Also, meters are only accurate to within plus or minus 15%.
To be honest, it is better to stick to one meter, keeping the other as a spare. Otherwise confusion sets in, as you have discovered. Unless you are on insulin or Gliclazide or similar it isn't vital. Just look for trends.