Portage is neither silly nor smart. Will do what will be told to do. By default is tracking stable packages unless you have specified otherwise either by changing universally the tracking $ARCH in make.conf or per package in package.accept_keywords file or directory. I would guess that you've got "sys-kernel/gentoo-sources" somewhere into package.accept_keywords in the past and forgot about it.
If gentoo-sources has not been manually overridden and portage complains about it then another package depends on the unstable version of gentoo-sources and trying to pull it, so you have to find which one is it.
