So it's probably true, there is no "real" ISO1600 on my A720 (and other powershot models too).
The message from allbest:
I think that iso 1600 on most of the powershot models is dummy. It is only gamma-correction of image that was shot with ISO 800 (my opinion). You can see result of those gamma correction in corresponding jpg-file... See discussion here SD1000 ISO Settings Error