Your points on 1 & 2 are interconnected and we can summarize that condo size is not official or standardized. Agree completely, but I would like to think when mixed en masse and looked at as a whole (excluding new dev batch closings that mess with monthly/quarterly trends), broad ppsf is the best tool we haveA foolish overreliance on $/sf is the hobgoblin of small minded thinking about the market. There are many reasons for this, among them ( somewhat random and disconnected)
1) Even in condominiums not all brokers use an "official" square footage. Many years ago we had a unit at 530 West End Ave. I listed it at the square footage in the Offering Plan. Another broker listed a similar (but smaller) unit as more square feet. We got an offer from a buyer which was low because they saw the other unit and "it was bigger so ours was worth less." No one who saw the two units would think it was larger. It wasn't larger in the Offering Plan. The only place it was larger was in the lie told by the listing broker. But that's the number which gets used for these reports. It's not totally uncommon to see listings with more square footage than prior listings of the same unit.
2) Even in condominiums there is no totally consistent measure of actual square footage of units. A few are the actual interior square footage of the unit. Some include the exterior walls of the building. Some include building staircases outside the unit. Many include the unit's pro-rata share of common elements (and you can imagine what that adds up to in buildings bragging about 100,000 square feet of "amenity spaces").
3) There are units selling at $600/sf and units selling at $6,000/sf. Variations in "average" or "median" $/sf are just as likely to represent changes in the mix of what was sold in whatever time period is being examined.
4) All we are hearing about is how blazingly hot the market is, especially in Brooklyn. Right now UrbanDigs Chart Room has price per square foot down 0.3% Month-Over-Month and 5.9% Year-Over-Year for Manhattan and down 4.8% Month-Over-Month and 2.6% Year-Over-Year for Brooklyn. So which is it?