I setup GSX boarding for 15 minutes but why does it take 16 minutes instead?
You might have timed it when you saw the baggage animation, but that's not what the boarding time refers to. The boarding time there refers ONLY to the simulated *passengers* boarding time, the baggage animation will always play separately from it, and always with the same fixed length.
The passenger boarding time starts when you hear a "ding" sound and a message saying that passengers are boarding, which will happen only after you open the passenger doors, and ends when you'll hear the 2nd "ding" sound, and the message saying boarding is complete.