I don't have proof but I have a (conspiracy) theory that AA/UA are creating an "artificial" shortage of seats to Europe this summer and metering out a few seats at a time at high fares to feed to the perception of fully booked flights. A few of the flights I have been following, keeps getting filled up in seat assignments slowly but the inventory stays stuck at Y3 B3 etc. I find it hard to believe that it is just seats getting assigned slowly.

Like all conspiracy theories, it has a strong possibility of being wrong, of course.