Gaming....please! Your going for an extreme to justify poor performace.
4 hours of STRAIGHT excessive use is crap. You make it sound like people will use it now and than. There are lots of heavy users that will get lots of constant alerts, these users will fall into the category closer got the 3 hour mark.
Though back to my point, 4 hours max for fitness tracking is a joke! This thing is made for the geeks that attend a gym, not people who are into adventure sports or anything related to endurance such as cycling, or even a two hour walk etc.
Now back to the geeks who visit the gym, imagine someone really into their fitness, goes to the gym in the morning for 2 hours, that's 50% of their battery gone before they start their working day. That is not acceptable. Even if they go at the end of the day, it will die in the gym. So back to having to charge it multiple times a day. And when apple say 19 hours, you believe that is what you will get??