It's not a hard concept to grasp, I'm allowed to ask questions aren't I?
Half the reason I'm asking the question is for the sake of seeing what kind of answers come up, more so than because I want an answer. And also because I feel it's specific inclusion comes up awful odd in questions involving the best/worst aspects of the Showa series, such as the 'Best Showa Movie' topic, where everyone has cited Gojira.
And the fact it's made in the Showa era is an irrelevant one as far as I see, because the first Heisei film was made in the 'Showa era' as far as actual Japanese history goes, and as far as I know, Toho's never used the terms 'Showa series', 'Heisei series', 'Millennium/Shinsei series' although I could be wrong.
Each era "ends" when Toho calls it quits until they restart it. Starting a new era once they pick things back up again.
So far this is the only post that I feel gives a decent reason.