If the average is $1500, then a number of transactions have been above average, so this isn't that unusual. There are a lot of reasons you might pay 5-10% above average: location, building quality and condition, how much deferred maintenance there is, historical or architectural significance, etc, etc.
An average is just that, meaning prices below and above. Chelsea is a much more desired neighborhood than midtown, which is one of the least desired neighborhoods.
So a 5-10% premium for Chelsea is actually on the low side, and isn't really a premium for the area as that sounds to be right on target compared to the average price.
If they paid $2,000 a sq ft then they would be paying a premium. This looks like a good standard rate for also an iconic building in Chelsea.
