Agreed that Apple needs to be a bit clearer on their software EOL policy.
It looks like currently Apple is using an "N - 2" policy, that is, they are supporting the current version plus the prior 2 versions. That would match with them dropping Lion support when Yosemite was released. Yosemite made Lion the "N - 3" release.
If I were a betting man, I would expect that Mountain Lion will drop off the support window when the next release of OS X hit the streets, possibly in a year's time frame since Apple does seem to be on a 1 year OS X release cycle.
Another point to consider - Macs released before 2007 can not run Mountain Lion or later releases. (I have one of them - a late-2006 Core2 Duo iMac). All other Macs since then can be upgraded to Yosemite. That also may factor into why Apple is dropping support for Lion - the hardware that requires Lion is in the obsolete hardware list.