A UIUID is a 128 bit unique number. A UUID's hash value is a 64 bit representation of that number.

Obviously we're losing information in the hash function. So it could well be possible for two UUIDs to hash to the same value.

Or consider a rudimentary string hash that simply sums the value of each character in a string. Do so and "abc" would generate a different hash than "xyzzy", but the same as "cab".