Devices appearing to “steal” other devices’ network identities (and therefore also their customizations) is an unfortunate side-effect of network segments containing Apple TVs, Airport Base-stations and certain other devices that implement a network service knows as the “Bonjour Sleep Proxy Service”. This can lead to symptoms such as multiple, identical entries for some devices or individual entries switching IP numbers with alarming frequency.
Here’s why this happens: devices that typically are inactive on the network for long periods of time (cell phones, tablets, iOS devices, etc) go into a standby mode to save power. Since they do eventually need to respond to network requests, however, some of these sleeping devices use the Bonjour Sleep Proxy protocol to allow the Airport Base Station, Apple TV (or other device supporting the sleep proxy protocol) to take network requests on their behalf. For this handoff to work, the Airport must masquerade as the sleeping device by temporarily “stealing” its MAC address; the former then wakes up the sleeping device when it receives a network request intended for the latter and returns its identity so it can respond accordingly.
This behaviour makes those sleeping devices appear to have the same MAC address as the base station (or an iPhone suddenly turns into a second Apple TV, etc), leading to multiple entries with the same MAC address but different IP numbers and generally confusing scan results.
If you have a network that displays this behaviour, there are a couple of workarounds:
1) add those devices that tend to get hijacked as ‘manual entries’ – this will supersede the dynamically scanned entry. You can then optionally enable the ‘Display current Ping status’ preference to see if the device is actually currently present on the network.
2) disable network services that use the Bonjour Proxy, such as ‘Wake for Wi-Fi network access’ and other wake-on-demand services.
3) disable the ‘allow duplicate MAC addresses & IP numbers’ option in the IP Scanner prefs, although in some cases you may then get an incomplete view of your network.
4) be aware that this phenomenon exists; understand it and live with it – Remember: the technology exists in order to save power and make the world a better place!
More information: search for ‘Bonjour sleep proxy’ will return a number of more-or-less technical results. Here’s a pretty good primer for understanding and troubleshooting the service: http://stuartcheshire.org/SleepProxy/