My intuition tells me that limiting a trip to two weeks isn't enough to define the low season; it's a bit of a smoke screen to think that duration is the key factor.
Here, in Spain, a short stay during Holy Week or regional festivals can cost much more than three weeks off-season.
The local calendar and specific events weigh much more on prices than the length of your trip.
I see a pattern where local demand and Spanish school holidays dictate the true seasonality.
To prove it, one would need to compare the cost of living for identical durations, but during periods including local festivals, regardless of international holidays.