By default, and a change from Pages v5.5, is that Pages v5.5.1 now defaults to writing what Apple refers to as a Single File format — which is compatible with non-Apple Mail and storage systems. It is essentially a compressed Pages v5.5.1 folder that has been renamed to look like a Pages document. This Single File format is not backwards compatible with Pages v5.2.2 on Mavericks, and probably not with earlier Pages v5 versions.
In Pages v5.5.1 on Yosemite, Apple has added some menu substructure that enables the user to convert Single File format documents into the traditional Package (folder) format that is backwards compatible with Pages v5.2.2 on Mavericks. Tested.
Because Pages v5.5.1 templates are in Single File document format (a departure from earlier Pages v5 templates), you must first generate a document in this format. Once saved, you can then use the following menu sequence, followed by a save operation, to convert the Single File format into the Package format.
Pages > File > Advanced > Change File Type > Package
If you open a Pages v5.2.2 document in Pages v5.5.1, make an edit, and save, it is written as a Single File format document, and not in the original Package format it was opened in.