I'm not sure if it's a bug or not, but it's not like automake 1.11 is masked ?
Unless it needs to be stabled first, then I simply misunderstood that eclass.
autotools.eclass is used in both stable and unstable, so only the latest stable can be declared there.
http://sources.gentoo.org/eclass/autotools.eclass?r1=1.98&r2=1.99