[enriquto]

I don't understand the concept of "slightly different hash". Aren't hashes supposed to be either equal or completely different?

[fizx]

You're thinking of cryptographic hashes. There are many kinds of hash (geographic, perceptual, semantic, etc), many of which are designed to only be slightly different.

[richardxia]

There is a class of hashes known as locality-sensitive hashes, which are designed to preserve some metric of "closeness".

https://en.wikipedia.org/wiki/Locality-sensitive_hashing