This is a public service announcement: depth and bpp are different.

Depth refers to the number of significant bits (usually colour-significant, i.e. R + G + B bits for RGB modes) per pixel. bpp, i.e. bits per pixel, refers to the number of bits used altogether for pixel storage. Ignoring alpha, the usual configuration of your framebuffer will be depth 24 (8 bits each for R, G and B), but 32bpp: 8 unused bits at the top. 24bpp and depth 24 means that there are no unused bits, and that four pixels will occupy 96 bits (12 bytes), and not 128 bits (16 bytes), as there would be in 32bpp. (Thankfully, no-one actually uses 24bpp in the real world.) That is all.