As you can see in the documentation here:
The value of this property is the same for apps that come from the
same vendor running on the same device. A different value is returned
for apps on the same device that come from different vendors, and for
apps on different devices regardless of vendor.
The value of this property may be nil if the app is running in the
background, before the user has unlocked the device the first time
after the device has been restarted. If the value is nil, wait and get
the value again later.
The value in this property remains the same while the app (or another
app from the same vendor) is installed on the iOS device. The value
changes when the user deletes all of that vendor’s apps from the
device and subsequently reinstalls one or more of them. Therefore, if
your app stores the value of this property anywhere, you should
gracefully handle situations where the identifier changes.
In short, the identifier for a particular vendor will remain the same if at least one app by that vendor remains on the device. Once there are no more apps left (or in the case of a single app, it is reinstalled), the identifier can and will change. As far as I know, there should not be a difference on iOS 6 vs iOS 7, so any difference you are seeing is coincidental.