Why does an RSA key (256 bits) have more than 256 bits of ascii characters?

As an example, I generated a 256 bit key pair. The private key (when saved to a file by puttygen) is: AAAAIGZcM73lV3Uk/psErZAYA0F+4xK78FtFoVi2fdypv92tAAAAEQDdOhyFJ6iV FSKDgjpj8+bRAAAAEQC2nAZD6DurGlj+DlPuB7tNAAAAEQCbHQFLFNR/URs4eu0o ZfPE

This is 132 ascii characters. Assuming only 6 bits per character (if the character set used is only 64 characters, that would make 132*6 = 792 bits, which is way more than the 256 bits that are supposed to be in the key.

So why the extra characters? Thanks.