Your father's father is called your grand-father, yet your father's uncle is typically called your great-uncle (or so it seems with anybody I converse with). Why the inconsistency?
There is no inconsistency. Grand is always used for "parent of a parent". Beyond that, great- and grand- mean the same thing. Your grandfather's brother is your great-uncle, grand-uncle, or granduncle.
It has been brought up in the comments that granduncle (written) is archaic everywhere, while grand-uncle (spoken) is archaic in Britain. I'm not sure if it's archaic in America too (I swear I've heard it), but you will not go wrong if you always use great-uncle.