gpg key change

[kr aid]

Hi, I'm pretty new to gpg and openpgp, but I have been using it casually
for a while to encrypt things occasionally. I have a question about
something that happened with an old set of keys, stored in a pgp .asc
file created by a windows version of pgp 8.1. The .asc file basically
contains armor ascii for both the public and private keys in one file,
like this:

-----BEGIN PGP PRIVATE KEY BLOCK-----
Version: PGP 8.1 - not licensed for commercial use: www.pgp.com

lQHpBDnK0XoRBADYZNigCSuy0tUEQwbIitqOD3Aer5VHCabX0ZnBPWl6go9kQm/t
......chunk of text removed.....
DAKPt5ea3+kjvQbXPFBVsyDQ4w7vJ2F+5zQcxjnij0cSqjOirkNMHAYD
=uAGK
-----END PGP PRIVATE KEY BLOCK-----
-----BEGIN PGP PUBLIC KEY BLOCK-----
Version: PGP 8.1 - not licensed for commercial use: www.pgp.com

mQGiBDnK0XoRBADYZNigCSuy0tUEQwbIitqOD3Aer5VHCabX0ZnBPWl6go9kQm/t
.........chunk of text removed
r8GijKr01256AJ9L9eqZaXbDvU02Px9OqqcHJavbxgCg/DLhqrcamZCvrdkJrsxp
k09bepE=
=HApt
-----END PGP PUBLIC KEY BLOCK-----





Anyway, I imported the keys by doing "pgp --import ./oldkey.asc" and I
was able to decrypt my old files. Later on, I decided to export the
armor ascii of both keys from gpg, but something curious happened. I
expected both keys that gpg exported to match the old keys in the .asc
file, but only the private key matched. So, for some reason the exported
private key was the same and the public key was different:

-----BEGIN PGP PRIVATE KEY BLOCK-----
Version: GnuPG v1.4.5 (GNU/Linux)

lQHpBDnK0XoRBADYZNigCSuy0tUEQwbIitqOD3Aer5VHCabX0ZnBPWl6go9kQm/t
....chunk of text removed.....
DAKPt5ea3+kjvQbXPFBVsyDQ4w7vJ2F+5zQcxjnij0cSqjOirkNMHAYD
=uAGK
-----END PGP PRIVATE KEY BLOCK-----

-----BEGIN PGP PUBLIC KEY BLOCK-----
Version: GnuPG v1.4.5 (GNU/Linux)

mQGiBDnK0XoRBADYZNigCSuy0tUEQwbIitqOD3Aer5VHCabX0ZnBPWl6go9kQm/t
....chunk of text removed....
ooyq9NduegCfS/XqmWl2w71NNj8fTqqnByWr28YAoPwy4aq3GpmQr63ZCa7MaZNP
W3qR
=uwWg
-----END PGP PUBLIC KEY BLOCK-----

What's interesting to me is that the top half of the public keys are the
same, only the bottom half changes. What I want to know is 
1. Why did this happen?
2. Will this be a problem on my end? To me it looks like the public key
that gpg uses is completely different from the key that I import in, so
will my private key still be able to decrypt from it?

thanks for any help that you can give me.

-- 
  kr aid
  k__raid at fastmail.fm

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