Thanks for the quick reply. This is odd behaviour, isn't it? I assume that I got seven entries in my 'My Devices' list because SityTrail silently added them (thus exceeding the maximum) and never deleted any. It's a surprise for a user, to say the least: it also implies that if you take your device into the field (without an internet connection) after an update, it may suddenly not work. I think you need at the very least a big warning on the update page in the store (although now iOS 7 offers silent update, so maybe that wouldn't work).
This morning I followed the instructions to delete the 'old' devices, and found that one of my devices apparently registered itself automatically (I could use it straight away) and two didn't, which was even more confusing. I also renewed the 'My Devices' page a few times, but so far only two out of three devices are shown. SityTrail is certainly working on the device that isn't shown (an iPad2). Not sure if this matters or not, but it seems wrong.
Incidentally, under iOS, I don't think one's device ID ever changes - it's wired in, so to speak. I don't know anything about Android.