Each pixel in your raw image is represented by a 2 byte integer. At 4928x3264 pixels, this gives 4928x3264x2 = 32169984 = 30.6 MB where a megabyte is actually (1024*1024=1,048,576) bytes. The D7000 raw format is a compressed format, so these 30.6 megabytes of data are compressed down to about 23-24 megabytes on your card.
To further confuse the issue, A megapixel is 1,000,000 pixels. The reason a megabyte is 1024*1024=1,048,576 is due to the way software developers think about memory and numbers and the fact that this is an even power of 2. Camera manufacturers like to use 1,000,000 pixels to mean a megapixel because it lets them put slightly larger numbers in their marketing literature. You D7000 is marketed as a 16.2 megapixel camera. If it were calculated the same way as megabytes are, it would only be 15.3 megapixels. Note: in recent years, hard drive manufacturers have taken to using 1,000,000 bytes as a megabyte for the same marketing reasons