But maybe I have calculated this wrongly.
Yes, you have. Polyphony is not only the notes you are just now playing, but also the releases of the former notes which can still be heard. If your sample set has, let us say, a 5 sec reverberation or more and if you are playing quickly, you can ear much more notes than the played notes, increasing very much the polyphony. For that reason the limits of the Hauptwerk free evaluation version may be quickly reached. Anyway, Hauptwerk is abble to correct the problem in real time, masking some releases if necessary and I suppose that you probably would not notice it. And you also can reduce the polyphony, if necessary.
Sorry for my poor english.