Why does my Nikon camera stop short of the selected number shots when interval shooting?

I found the passage below in the Reference Manual and was wondering if the problem might be that I did not set the interval long enough to allow the camera to record some images to the memory card: “Because shutter speed and the time needed to record the image to the memory card may vary from shot to shot, intervals may be skipped if the camera is still in the process of recording the previous interval.”

Yes, this surely is the case.

I discovered myself conspiracy about shutter speed recently. I needed to photograph long series and found out that camera takes photos gaplessly if I set interval to 32 seconds and exposure time to 30 seconds.

I was extremely disappointed that two cameras from different brands expose not for 30 seconds but for 32 seconds! It looks like cameras set the exposure using power of two divider or multiplicator to get exposure times other than 1" to get exposure times closest to traditional 1/125, 1/500, 13", 1/15, whatever, but I am not sure in that.