How is it possible?

This weekend we were lake bound the entire time. A friend of mine who just got done redoing his whole stereo provided the tunes for the beach. Now pay attention this is the important parts. On Friday he played for 4 hours @ 1/3 to 1/2 volume. Skip Saturday. Sunday he played for 9+ hours at the same volume. in all of this playing we had two surf sessions that lasted about an hour and one lap around the lake that was twenty minutes. So a total of 13+ hours of play time with only 1 1/2 hours of alternator charging time. His battery bank is two 6v batteries. The system is all Exile and consists of 2- 800.4`s /1- 2500.1 /and a harpoon. so four amps total.

Does this really seem like it should work? I thought for sure we would be switching to his reserve battery bank. What are your thoughts? Am I under estimating the batteries or is his system really that efficient?