Just as a bit of background:
Updating via LiveUpdate and updating via the GUP/SEPM are two separate operations that have their own schdeules.
Updating via LiveUpdate works in accordance with the LiveUpdate Schedule you have configured in your LU Policy.
Updating via GUP&SEPM works in accordance with the Heartbeat Interval assigned to the group to which the client belongs.
In SEP11, both operations will attempt to update whenever their respective schedules hit. It's all a matter of luck as to which operation hits it's next scheduled run after new definitions have been released. The first to do so will be the method the client uses to update. As both operation types (can) work on intervals, some clients will grab the latest definitions via LiveUpdate, while others will see defs via the Heartbeat and download them via the GUP/SEPM.
Just be aware, in SEP12.1 a new setting was added to the LiveUpdate Policy -> LiveUpdate Schedule area called the LiveUpdate Skipping option. This allows you to configure thresholds for when clients will attempt to use LiveUpdate (the default thresholds are "more than 2 days out of date" and "out of contect with SEPM for 8 or more hours"). This setting means you can enable all the LiveUpdate options, but that client will not perform a LiveUpdate unless it has been unable to contact a SEPM for 8+ hours and it is more than 2 days out of date (both conditions must be fulfilled)