Then in that case because some apparent pairs of letters are in fact a single letter in some languages. Per the DZ wiki page someone else has given, DZ is a distinct single letter in hungarian and slovak at least.
Double letters being a single letter isn't rare, such as ll and dd in welsh. In a strange sense it arguably occurs in english in th and th, which I could argue are double-letter representations of single letters, which were originally eth and thorn
