Finally, I got to drive the car to drain it all the way down to near empty with left side down to 1.3 liter while right side still at 3.1. I supposed it does not take much deviation in the float lever to get a 1.5l difference. The car has never had its right tank opened up, so it must came from factory that way. Any ways at least I can say now the problem is fixed.

Thanks to everyone's help!