In my experience that comes from slight differences in the placement of the pads.
If on one leg you are right on spot and on the other not -> different response.
Try moving the electrodes a bit, I assume we humans aren’t 100% symmetric 😉
Other reason I can think of: worn out pads, but that is unlikely if you bought the device new.
On my Compex device I can adjust the electrical signal for each canal: this also helps to fine-tune the contractions.