ABCDE only describes output balance, however because the 5-5 consists of entirely 2-2 splitters, if one side is balanced then the other side must also be balanced. This is also why only the 4-4 works. If you instead combine 1-5 with something like 2-4 or 1-4, the result wouldn't be input balanced. The 9-9 and 10-10 were also made using this method, by combining 8-8 with 1-9 or 2-10.