I don't have the book here but...
The age-era classes make it hard/impossible to use one bike for two classes - basically, the rules are written such that a bike can only be used in one age-era.
However, Evo is not an age-era class, so it stands that bikes can be used in their age-era AND Evo. The 'proper' Evo bikes can probably also enter pre-85, for the same reason.
Could be wrong, but something to think about/look for while reading the book.