I am finding the choice of braille characters confusing: I would have assumed that one would use the 8 dots to represent the 8 bits, and that seems to be happen

I am finding the choice of braille characters confusing: I would have assumed that one would use the 8 dots to represent the 8 bits, and that seems to be happening for the very high values--like 0xfX--but then 0x81 should have two dots... what is the logic? (I did verify that Unicode seems to have a "full set", as I could have seen them deciding to only encode ones that were "used" or something, and maybe braille had been designed explicitly to be readable upside down or something and so only used half the options, but I found a chart mapping the characters for all 256.)