It’s essentially a non-trivial problem to determine what a user’s baseline mind state is. If you’re not at your baseline during calibration, then the algorithm to determine calm vs stress will be inaccurate. That’s why you’ll get calm scores much more easily if you begin a session in a stressful state.
Stressed out baseline = easier to reach a calmer state during session, harder to reach an active-mind state.
If you start your session already in a calm state (which can be very easy for an experienced practitioner to get into; just getting into position and closing their eyes can be enough to put them straight back into their meditative state), then it’ll be very difficult to get calm scores.
Very calm baseline = harder to reach a calmer state during session, easier to reach an active-mind state.
I recommend just experimenting with how you calibrate.