battery percentage indication not right.

Bafang mid motor with stock display. Why display indicates inaccurate battery percentage?

I would never expect SoC% to be accurate.

Even with dedicated pack monitors costing hundreds, very difficult to keep them calibrated within 8-10% of reality.

because it is based on voltages of cells.
real SOC uses Columb count , meaning charge going out and going into pack.
circuit for this can be complicated with dedicated chip.
Stromer ebike or Bosh ebike or Yamaha EBIKES would not use such cheap ways to measure SOC.