This happened late April/May already.
There is a simple explanation: Stamp is liquid. It's the easier one of the lot if you want to get fiat out faster.
Gox wait times go much much worse when the herd heads for the door.
So I speculate that a part of the herd is heading for the door, choosing the fast lane*, which is Stamp. Hence the differential you see.
* That doesn't imply certainty of a short term drop. There are other forces at play.
Coinbase has to dump their extra coins somewhere (stamp) when they get too many flooding in over what they have sold themselves
remember back when coinbase had those buying freezes when they didn't have any more coins and they would only sell if you took a chance at the price in 3-4 days?
They haven't had any shortage of coins anymore.. and the only delays are the AML law limits not coins (they lock in price but delay delivery on large purchase, and very low instant buys)
I've been studying coinbase as much as any outsider can since last spring and it is pretty evident now that they are THE market makers now.
They can actually trade at a small loss on stamp if they have to since they make up a substantial arb on their fees vs stamp's fees and their spreads vs stamp spreads.
they are ripe to get crowded out of this little lucrative racket soon...but I do not think the price of btc can sustain any spikes upwards. If someone wants to compete with coinbase, they will just settle into the same range and pattern coinbase established and look to undercut them on fees. There is no reason to try to raise the floor while there are still liquid exchanges out there, it would be a failed unsustained pump. If you had enough coins to try to slam the price, coinbase is big enough now to call the bluff, absorb short term losses and grab cheap coins.
And remember coinbase makes their money on fees, so they just want to promote more velocity with the merchants & customers using their payment process.
Ironically, the only fear coinbase has is if the same merchants and customers begin to feel safe with the very stable price coinbase is providing and avoid coinbase and deal directly.
If that happens to any large extent we go back to the wild volatility since coinbase steps away during major volume spikes and throws up $30 spreads during those storms.
The question would be, could there be enough frequent storms of volatility to break coinbase?