Rear wheel bearing dirty so it will not center. Side sensor stuck or dirty so it thinks it proximate to something. Then again, what does it matter if it won't run straight if it covers most all the floor over time.

Just a few ideas. (1) The wheel to the outside of the circle has a broken speed sensor. Could be a cracked magnet disk that is on the back of the motor. Do a visual inspection on the magnetic disk. You can see it on the back end of the motor.The fix is a new Left wheel assembly. If you run the built in test (instructions all over the place, you tube and more) You can see the left wheel turn very fast, like maximum speed, The right wheel will be slower. The fix is a new Left wheel assembly. (2) Mother board is not reading one or the other wheel sensor. This is a last resort thing. Easy to trouble shoot with an oscilloscope. (3) Right wheel fouled with crud causing it to run slow. Clean it with another real vacuum or blow out with compressed air. (4) Front Sensor covered with junk. Blow compressed air between the front bumpers and the panel inside.