As per: http://www.gentoo.org/proj/en/releng/#doc_chap5 Releng has two signing keys. One is used for automated builds, and the other one is used manual builds. The handbook only references one key presently.
How do we know which is used for what builds? AFAIK releng just pushes out autobuilds, so any other signing keys are superfluous.
You have to try to verify the signature (or use GPG at least) to get what the signing key was. That's why I think both keys should be listed on the page.
Added the second key to the --recv-keys command. Thanks for reporting!