I think you are right the tub must be slightly uneven, maybe so it drains properly? Sounds like you may need to shave the stringer.
In addition....... the back left to front left is level and the front left to front right is level after putting a 1/4" shim under the front right. But the back left to back right is still not level. The level shows that the rear right needs to go down 1/8-1/4"(even though the stringer itself is level). Just to see if it would help, I moved the right of the stringer down 1/8-1/4" so it is no longer level (hoping the top back of the tub would be level) and the back right of the tub will now not set down on the stringer. No matter what, I can't seem to get it level. Even if all three edges are level, the back right corner is off. I am beginning to wonder if the tub is warped.