Eight bits have 256 combinations, but if you know that eight out of eight bits are zero, than there's just one combination possible: 00000000. Thus knowing how many bits are 0 and 1, can reduce the amount of possible combinations extremely. The more information there is about a data block, the less combinations are possible. Not just knowing how many zeros and ones there are but knowing more; how many groups of each: 00, 01, 10, and 11 there are (or larger groups), reduces the amount of possible combinations even more.
Next are all 256 combinations of 8 bits, in order of the ratio 1/0