Ah! This is why: They come out to round numbers once you work in colour values! 320*200 pixels = 64,000 pixels. Not divisible by 1024.
BUT 16 colours = 4 bits per pixel, or 256,000 bits needed. Which is 250 KB of RAM, a whole number. Not exactly on 256 KB, but a) if it isn't separated from system RAM it doesn't have to be, but it would be weird if it wasn't an even number of KB.