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.
The size of a lossless compressed NEF file will vary depending on the type of subject. For a very detailed subject with many different tones, the amount of compression possible may be minimal. The figure in the manual is "conservative" - in the same way that you generally get more shots than the figure for shots remaining in the camera display.
I guess I figured that the figures would be based on the same series of images for comparison purposes. The manual suggests there is no real advantage to compression other than the resulting buffer capacity.
Scott Chapin Powder Springs, GA, USA Nikonians Team Member
As usual, the manual - at least the English version - is quite misleading, even if it probably is literally correct. Nikon's are some of the worst in this regard, at least as far as users are concerned.
_____ Brian... a bicoastal Nikonian and Team Member
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I always use lossless compressed RAW. My experience is that after formatting my memory card the shots remaining agrees with the tables in the manuals. In practice I get over 50% than shots than the shots remaining indicates. This has happened consistently with my D300, D800e, and D4. That appears to be similar to other Nikonians.
As other replies indicate, Nikon seems to be giving very conservative capacities based on a worse case of negligible compression instead of using the expected file sizes used in the table.
Gary in SE Michigan, USA. Nikonians membership - My most important photographic investment, after the camera. D4, D810, D300 (720nm IR conversion), D90, F6, FM3a (black), FM2n (chrome) YashicaMat 124, Graflex Speed Graphic 4x5 My Nikonians Gallery & Our Chapter Gallery