They specified an invalid SHA512 algorithm instead of SHA256.
This looks like a typo. Although it does not do honor to such a site.
I rechecked the hash with the SHA256 algorithm and it is really correct:
Meh, in fact it was predictable that all situation will resolve itselfe in such way (though i bet that this was because OP hashed seed as a string)
I am surprised that many people used this site and no one noticed the typo. It turns out that no one checked the hash or people somehow guessed what algorithm to use.
Ahahahahahahahaahah.
this is typical. I would say more - this is
normal, for average people to not pay attention to such things. So, OP despite he is not so knowledgeable about sha256/sha512 (there is no need to use "heavy" 512, while we have strong and more easier sha256) he is enough attentive to check his seed.