How can a public key contain several public keys with GPG (or what am I doing wrong)?

I'm setting up some Ubuntu servers. I received from another sysadmin a key to be added (call it somekey.pub) for apt package verification on in-house packages. Adding this key with apt-key add somekey.pub results in TWO additional entries showing in apt-key list, each with a "pub" line and a "sub" line. (The "uid" line on both new entries is the sysadmin who gave me the key.) How is this possible? Inspecting the key with less shows: -----BEGIN PGP PUBLIC KEY BLOCK----- Version: GnuPG v2.0.22 (GNU/Linux) (41 lines snipped) -----END PGP PUBLIC KEY BLOCK----- I also used gpg to create a keyring containing just that key (using gpg --no-default-keyring --keyring ./somekeyring.gpg --import somekey.pub), so that I could put the keyring in /etc/apt/trusted.gpg.d/ on other Ubuntu servers rather than running the apt-key add command. Inspecting this keyring with gpg --no-default-keyring --keyring ./somekeyring.gpg --list-keys confirms that it has two keys. Here is the output, munged slightly: pub 1024R/4AAAAAAA 2018-08-31 uid Joe Sysadmin (Ubuntu Dev Repo Key) sub 1024R/9FFFFFFF 2018-08-31 pub 2048R/BAAAAAAA 2018-08-31 uid Joe Sysadmin (Ubuntu Repo Repo Key) sub 2048R/1EEEEEEE 2018-08-31 This is my first foray into GPG and apt keys, so I may be missing some simple basic piece of information, but I would expect that the single public key block in somekey.pub would only contain a single public key - so the above results surprised me. Where is the documentation that will allow me to make sense of this?