A Stitch in Haste

Befuddlement:

The housing bubble may be deflating in the rest of the country, but the New York City real estate market is hotter than ever.
...
The average price of condos is up by 22 percent, while co-ops jumped 5 percent.

For those who need a primer, the basic consequentialist difference between a condominium and a co-operative is that with a condo you can (pretty much) buy and sell the apartment at will, while with a co-op any sale requires approval of the board of directors. Mainly for this reason, condos will, ceteris paribus, trade at a premium to co-ops.

But why should rising prices, whether from a bubble or not, result in such a wide differential between condo and co-op price increases? If condos sell at, say, an average premium of 25% to co-ops, then one would think that this premium should, in percentage terms, stay relatively constant as the market rises and falls. Whence comes this "22% v. 5%" expansion of the premium?

I suspect there's a bit of multicollinearity occurring here. Perhaps condos tend to be newer buildings, or larger apartments, or more concentrated on the luxury end of the spectrum? Something other than the basic "condo premium" has to explain these disparate increases. Otherwise there would have to be a bubble not just in real estate but in the condo premium itself. Is that likely?

Any thoughts?