That would normally be because of your other recordings set at the same time mean that the tuner you are using is required to make the recording.
It does it 15 mins before because the tuner is required to look for the broadcaster accurate recording start recording signal.
What recordings and timings did you have set with times that overlap the specific recording.
Look at your list of recordings and post the channel plus times that you had, that will let us analyse which MUX you were using also post the channel you were watching at the time.
Chances are your recordings were set to use 3 mux and that the the channel you were watching was on a 4th Mux. 3 tuners can only use 3 mux at the same time though each tuner can use 2 channels from the same mux. The last recording due to start was the straw that broke the camels back, it needed your tuner.
The box cannot possibly know what other channel you might be watching live when you set recordings.