It's not just decoding the base64, it's actually encrypted somehow as well in a way that the site knows how to decode.
I played with the encryption on the same page with similar inputs and got totally different outputs:
AAAAAAAAAAAAAAAAAAAAAAAAA encrypts to uUpqkh5hTLhci/Ahlu3E9GoJkwevuSjYtCoeJC7obhM=
AAAAAAAAAAAAAAAAAAAAAAAAB encrypts to VTyczwRvJaaTOM5LtG5ZcKE9s5j4VtyxBdig2uI7sYA=
AAAAAAAAAAAAAAAAAAAAAAAAC encrypts to ihumZYiqrmFC5xKIem1dc/hAuix7rUoBFgYAAReR8Iw=
They're totally different, not just the one letter off