Tag History Value Mode and Wrong History Values

I’m wondering what’s really happening when I set a Tag History Value Mode to Analog.
I tried what follows

  • set a motor_rpm Tag History Value Mode to Discrete: plots on the Easy Chart component as well as points I can get with system.tag.queryTagHistory are correct, i.e. they just look as I expected;
  • set the same tag Value Mode to Analog: plots and points have nothing to do with the history of the tag at all (values are really far from the rpm motor values I saw in realtime).

I read this part of the Ignition manual:
however cannot grasp the reason why my results are not as expected.

(Ignition version: 7.9.9 (b2018081621))

Thanks in advance, regards