When is Forteana not Forteana?
I would say it relates to the boundary or distinction between:
(a) the observation or apprehension of something anomalous versus ...
(b) the interpretation one applies to describe / frame / identify / explain the anomaly.
My take on Fort emphasizes the role of puzzling evidence that contradicts established knowledge / doctrine / etc. Such evidence may be circumstantial and merely suggestive (i.e., (a) above), or it can be attributed weight by contextualizing it in specific terms contrary to common knowledge or even common sense (i.e., (b) above).
Position (a) can involve nothing more than 'woo' - i.e., finding oneself in the 'uncanny valley.' Position (b) involves traversing the uncanny valley and planting one's flag (however seriously or finally) on one of the hills (figuratively - explanatory vantage points ....) lying beyond.
Going back to the villa example ...
Simply discovering a ruined villa with features suggestive of Greco-Roman style sitting isolated on a beach is an instance of (a) - Weirdness Typifying Forteana (aka "WTF").
Going further to adopt an explanation for this WTF - e.g.:
- A concrete building miraculously transported to the beach by a hyper-mega-tsunami;
- A substantial structure teleported to the beach by aliens; or ...
- A remnant of Atlantis uncovered by the tides
... is an instance of (b).
If one considers WTF-level oddities to be Forteana, the villa is Fortean regardless of the application of an extraordinary explanatory context (or face value validity thereof) simply because it induced some 'woo factor' in the given observer(s).
If one considers nothing properly Fortean until it's contextualized with an extraordinary explanation, the mundane historical explanation de-escalates the villa to the status of something odd which challenges no accepted knowledge except where one should build one's seaside hotel.