Why are some ebuilds only marked ~x86?

This is because Mark (the lead developer) only has x86 machines. Per Gentoo policy <http://www.gentoo.org/proj/en/devrel/handbook/handbook.xml?part=2&chap=5#doc_chap2>, one may only keyword ebuilds for arches that they have personally tested.

Merge requests will be accepted for keyword addition(s) if more than one person can attest that the ebuild(s) in question do, in fact, run properly on the arch(es) in question.