MyISAM compression is based on Huffman
compression. In his article from 1952 Huffman proved that his
algorithm uses the least possible number of bits to encode a
sequence of messages. The number of bits assigned to each
message depends on its probability to appear in the sequence.
Huffman did not specify exactly, what those "messages" are. One could take all possible values - say of a table column - as "messages". But if there are too many of them, the code tables could become bigger than the uncompressed table. One would need to specify every possible value once and the code tree with its indexes and offsets. Not to forget the effort to step through big binary trees for every value and - on the encoding side - the comparison of each value against the already collected distinct values.
The usual way to define "Huffman messages" is to take the possible 256 values, which a byte can express, as the "messages". That way the code trees are of limited size. On the other hand, the theoretical maximum compression is 1:8 (12.5%) in this case.