It's not very nice to hear: we already know that the new MacBooks Air did not incorporate SSDs 50% faster, as some rumors pointed out, but the annoyance that Apple seems to have kept a version of SSDs slower, from Toshiba, in certain lots.
Apparently, most 11-inch Airs use a Samsung SSD (SM128C) that reaches ~ 260MB / s for reading and ~ 240MB / s for writing, while several of the 13-inch incorporate one from Toshiba (TS128C ), with speeds of ~ 210MB / s and ~ 160MB / s, respectively.
For many people this difference should not have great effects on the daily use of the machines, but it sucks to think that you can buy the laptop with one or the other, in the dark since Apple only allows you to choose what its capacity will be.
If you want to know the model of yours, go to the ?Serial-ATA? section of the System Profiler, accessible through the menu of the Apple, in the upper left corner of the screen.
(tip from Alexander Novarro, via runaroundtech.com)