It is nothing new that one can use multiple Android devices with a single Google account. Switching on a new device for the first time asks whether one wants to store one's data with Google, which then would always sync "some stuff" to the Google servers, basically
- some application data (if the apps support it explicitly)
- Wi-Fi passwords
- browser bookmarks
- a list of the apps installed from Google Play
- words added to the dictionary used by the onscreen keyboard
- most of your customized settings
Details might be found in the Google Dashboard. Relevant questions here covering those issues include:
- What info does Google backup?
- What exactly is synced with Google?
- How does Android sync the WiFi Profiles?
The Developers API on Google Backup gives some further insight on how the backup stuff is supposed to work (and several questions here show how it really works -- that is, sometimes it does, sometimes only partly, and sometimes not at all). Aside from reliability and the fact that not everybody wants his private data in the cloud (and even mentioned API reference2 warns: Android makes no guarantees about the security of your data while using backup. You should always be cautious about using backup to store sensitive data, such as usernames and passwords.), my main question is:
Having backed up data from multiple devices using the same account:
- what would happen to a factory-reset device being used that way before? Would it be recognized, and have only those things restored which have been used on it before?
(device-identification could e.g. take place e.g. via IMEI (but not via the Android_ID, as that might be gone with a factory reset) -- and this could be the reason for the behaviour described in Nalum's answer)
- what would be restored to a (new/factory-reset) device you just initialize for the first time with this Google account?
(if devices would be identified with backups in the Google account used, this could trigger a special action for "new device", e.g. "restore all, device changed!" -- or "restore all from the no longer connected device X, as it was probably replaced!" -- but stick to "restore only what was on that device" in case of a factory-reset)
The deal is: If one has multiple devices, they are often used for specific issues, so one does not want everything on all devices. As I've seen no way to chose which data to backup (e.g. to exclude those "sensitive data" we have been warned about: WiFi passwords would belong to that category), I assume there's no choice on restore either? So how is this handled?