Can never be sure what Nikon was thinking but the following would be my guess.
The numbers they are giving for the lossless compressed files is just a conservative specification. Note they give single photo file sizes which they list in the footnotes as approximate. For any given compression algorithm there is no way to guarantee the amount of savings that you will get for an unknown image. In act, it is possible for a compression algorithm to produce a larger file that the original. Of course, any decent algorithm would check this and just write the original uncompressed in that case. In any event, if you need to take x number of shots, nikon is just giving you the most conservative number of cards you need to have.