Many wallets are not exactly full. 24 bit color is three 8-bit bytes, one byte for each of the Red, Green, and Blue CCD channels, to describe the color of each pixel in the image. 30 bit color uses 10 bits for each of the three primary RGB colors. In binary numbers, each bit is a power of 2, meaning that each additional bit doubles the maximum size of the numbers that can be stored. Same concept in decimal numbers using powers of 10, each digit there allows numbers 10 times larger to be represented.
Scanners detect light intensity corresponding to the density of the original.
More film density lets less light come through, or more print density reflects less light. The CCD sensor measures that resulting light intensity. The image RGB numbers stored are proportional to intensity values in the original. Basically the human eye responds to brightness in a logarithmic manner. The human eye does not perceive twice the intensity as being twice as bright.