I haven't really thought of it in this detail (other than FF to FFX being the era where it "felt like" Final Fantasy to me, but that is pure opinion.), but I guess that FF to FFV could be seen as "The Crystal Era", as Garland basically pointed out.

FFVI to FFX still feel like Final Fantasy to me, but they indeed aren't part of The Crystal Era - they may be more a technology meeting fantasy era, a "Science Fantasy" almost - which is ultimately the reason I don't mind (and in fact, like) fantasy stories having technology. Final Fantasy IX is kind of the exception to both of these, but that makes sense, as it was going back to the series' roots in various places - it was the "Nostalgia" Final Fantasy.