One reason for the discrepancy might be that the app records time spent asleep, not time in bed. Taking this into account, if you still feel Sleep Tracker has shut off prematurely, this might be caused by insufficient phone charges. Please be sure that your phone is plugged in while using Sleep Tracker.
At the same time, the background sound in the surroundings during sleep may also affect the accuracy of the sleep tracker. To ensure the accuracy of the data, please try to place the phone at an arm's length away from you, point the microphone at you, and try to reduce the ambient noise while sleeping.