Yes you are correct. Each file size varies with the scene as well as the compression used by each camera.
On my D7 for example, the output varies anywhere from 1.6Mb to 3.1Mb on the lowest compression jpeg setting. Highly detailed and color scenes will usually result in bigger file sizes. The camera doesn't know this ahead of time so it made an average worst-case assumption, and depending on what you shoot, you could have a larger total number of shots than what the camera originally estimated... On very large storage like the microdrive, it could be off by by close to 200 frames with studio shots on monochromatic backdrop !!!
Without compression RAW output is usually the closest number of bits put out by a 2560x1920 CCD of a camera.
[Edited on 8-7-2002 by NHL]