How a spring could have differential force inboard and outboard is not understood. May be you feel so.
I guess it is probably hydraulic valving rather than springs that creates the different stick force but either way its quite obvious if you hold the stick from above with your other arm and move it side to side, you can clearly feel the different resistance. The fact that nobody notices (as intended) in normal operation shows how well judged the mechanism is.
Here is a scan from "The Airbus Fly-by-wire".
