The WEP SSIDs are the AP to AP communication for the meshing. It uses a 1mbps/6mbps per band (2.4/5) signal, which is kind of a bummer because that takes up a LOT of airtime. If you have meshing turned off, it is supposed to disable the SSID from being broadcast, but for a while it wasn't. It may have been fixed by a firmware upgrade, I haven't looked in a while. Even though the communication is just some basic info about the mesh, WPA2 should really be used and not WEP. It's possible there is an attack vector in there yet to be undiscovered, but with such a weak keyspace, it would be pretty easy to start looking. Edit: I'm pretty sure the data within the mesh communication is encrypted, but, still.
TLDR; It's the meshing communication between access points.
As a side note, there really isn't much different in range between access points (even between vendors). They are all locked to a maximum output RF strength. The difference is probably in the antennas - the MX65W whip antennas probably work a bit better in your environment than the integrated radio on the MR32. You can view the RF coverage charts from the documentation to show how each AP/antenna type distributes the signal. The mounting plate also has a 5% or so reduction in signal going that way, so YMMV.
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