What does geohash_decode do?

[euglv]

SELECT 
    geohash_encode(57.64911, 10.40744, 5), 
    geohash_decode( geohash_encode(57.64911, 10.40744, 5) )

returns: u4pru and

-82.7929688,-179.5605468
-82.7490234,-179.5166015
-82.7709961,-179.5385741

As I understand last line of geohash_decode(hash) result should be central point of the hash, but it isn't.