Originally posted by zvon Maybe that is the case for other Pentax cameras too, I do not know.
Yes, that's right. Image files will be all different sizes depending on compressibility (which is in turn dependent upon image content, ISO/noise, etc.), so the camera gives essentially a "worst case" estimate.
I have always been able to fit a lot more shots than the initial empty-card estimate, often 50% more. As you get closer and closer to full, however, the number will be closer and closer to correct -- the camera continuously recalculates the number based upon actual free space, not prior shots vs. remaining shots. So if the camera says you have three shots left, believe it!