No difiere ... serian 32 bits para 4 bytes, 3*8=24.
1Ch 2 the number of bits per pixel, which is the color depth of the image. Typical values are 1, 4, 8, 16, 24 and 32.
"RGB color (24-bit) pixel values are stored with bytes in the same order (blue, green, red) as in the color table."
http://en.wikipedia.org/wiki/BMP_file_format