Hi,
I now can answer my own question, as Santa provided me with the TabuTronic device.

As assumed, the WHY? score after a dubious move you entered is the resulting one, should you neglect the Tutor warning and confirm your initial choice. The sequence is:
- Tutor is on, it is your turn to play
- you enter your move
- the device computes on its turn; resulting in its own "S" score + intended move
- if the score shows a significant gain from the computer's point of view (POV), Tutor asks 'are you sure?' - thus suggesting you should better use the Take Back key (of course you may confirm your choice, using the Move key).
If you wonder why Tutor gave you this warning, you can ask using the Why? key; then Tutor will voice the reason why, and additional pushes on the Why? key provide further explanation, including the potential resulting score from the player's POV, which is -S.

Following a computer move, there is no more score computation: If you request it before entering the computer's displayed move, then it is the "S" score from the computer's POV; and once you actually moved the computer piece, requesting the score displays -S: same value, negated to reflect from player's POV.

MfG,
Tibono