The Tesla rep told me that there are 11 sheets of batteries.
That one sentence is the only true statement. The rest is highly doubtful.
Standard mode runs 2 sheets at a time with performance mode using 3. When a sheet is depleted to a certain level it will switch to other sheets. So with different sheets being used to different charge levels, when charging is finished, not all the sheets will be at the same charge as the highest charge sheet will determine when you are done.
The Tesla Roadster needs all 11 sheets to reach the nominal 400V that runs the HVAC and, of course, the motor. As far as I know, all sheets are in use at all times. I was told that two sheets were used in early model ESS systems to run the 12V subsystem, but that not only doesn't add up, it was also shot down on this forum. Seems like the Tesla reps do not understand the technology all that well, because I've heard many things directly from "Tesla" that turned out not to be true.
It's entirely possible that charging in Standard Mode works on 2 sheets at a time while charging in Performance Mode works on 3 sheets at a time - there's certainly the option of charging only parts of the battery pack. As I said, though, the whole thing is engaged when driving.
The real reason that cells become unbalanced is
not because the Tesla firmware uses only part of the cells - they're using all of them - but because the chemical reactions in each cell are impossible to perfectly match. Even with liquid cooling and advanced firmware, it would be impossible for 6,831 cells to perform exactly the same at all times, whether charging or discharging. It seems like your Tesla rep was looking for a complicated explanation when chemistry and physics are enough to explain why this occurs.
Occasionally charge using range mode, possibly several times back to back to get all the sheets at the same charge level. I did this to mine and got 10 indicated ideal miles added to my range.
It probably isn't necessary to charge in Range Mode if you want the battery balanced. It should be sufficient to charge in Standard Mode, wait an hour, and Top Off if you want to allow time for the cells to balance. But I admit that there might be something I missed here.