how is the sha fingerprint generated?

[shawn wilson]

% cat ext_rsa.pub| sed -r 's/.*(AAAA[^ ]+).*/\1/' | sha256sum

  ~/.ssh swlap1
d4bf8b06f2d9d9af7a11583a5367205ed310a84f0dee68d062e2ddca1e85c3ff  -
 % ssh-keygen -lf ext_rsa.pub

   ~/.ssh swlap1
8192 SHA256:FgrfxmdjTM/j4wwRa7nVdPSUaJdqHYMJtJ6aciPl9ug swilson at swlap1 (RSA)

Why do those differ and how would i generate the equivalent (mainly
just curious)? I've also tried base64 and a few other substitutions at
the end and I can't get them to match (probably would save time to
just look at the code, but...).