http://u.qdnx.org/file/-20161213T234321-n3h97vl.html

@csaurus A message sent to multiple !GnuPG (or #PGP) recipients uses a symmetric key generated for that session to encrypt the message, and then encrypts that symmetric key with the public key for each recipient, creating multiple encrypted key packets. When you receive such a message your GnuPG finds the packet with the symmetric key encrypted to your public key, decrypts it with your private key, then uses the decrypted symmetric key to decrypt the body of the message. The packets encrypted to other public keys are ignored.