Do AppleTVs show up in AirPlay by proximity?
The 6.1 update added airplay discoverability via bluetooth to the Apple TV, and it sounds as though this is what you are seeing, especially since the distances you mention are not dissimilar to bluetooth ranges.

From Macrumors:


  This change allows Apple TV to search for connectable devices over
  Bluetooth, bypassing Bonjour and making it easier for education and
  business administrators to set up iOS device and Apple TV pairings.
  Previously, Apple TVs could have a difficult time discovering devices
  on networks that might block Bonjour.


At the time, it required an iOS 7.1 device and did not work with a mac, but apparently this feature was added in Yosemite:


  Unfortunately, this Bluetooth Discovery feature has not been available
  for Macs, which is why your iPhone/iPad can see the Apple TV and your
  Mac cannot. Fortunately, with the latest beta release of Apple's
  Yosemite OS, this appears to be working now (Hooray!)


iOS 8 added a peer-to-peer airplay system for third generation Apple TVs, however since your Apple TVs are on the same network as your other devices it seems less likely to be what you are seeing (though it could be useful if you have guest presenters).