Why does my camera have different dimensions for the same RAW and JPEG file?

I take pictures as NEF+JPG on a few occasions. I noticed that the pairs of photos will come out as follows:

JPEG dimensions 4288x2848
NEF 4310x2868

Any idea why this happens?
BTW Nikon D90 at 12 Megapixels

Added (1). The JPEGs are are Large, Fine.

My understanding is that the RAW means: No processing has occurred to the image yet and the color ranges and balance are untouched. After processing you get the JPEG.

If you see picture-taking as a series of steps, then the camera should be able to take more RAW pictures per second than say JPEGs, and that'd be because it's skipping the processing step.

Does it now.

I never looked to see if mine had slightly different sizes.

Since RAW uses all the data collected by the sensor, I guess there could be times when additional pixels could come into play, especially if you are not shooting using Large size, Fine quality JPEG's.

From last shoot:

RAW = 4288 x 2848
JPEG Fine = 4288 x 2848

No difference on my D300 12 mp sensor