Y-axis calibration on Workbench

Hello,
I tried today to print some testing circles with a diameter of 20mm. My problem is that I get a good diameter of 20mm on the X-axis but "little" variation between 18-21mm on Y-axis so, my circle appear a little flat ...
I saw that it could be due to the belt tensioning or misaligment but they seems nicely aligned and I think that belt tensioning is good.

I've more or less the same tensioning onto the two belts of the Y-Axis, but I don't exactly know if those belts must be really hard or a little smooth. What about the belt between the stepper motor and the Y-Axis ?

Thanks for helping...