1. Some folks cheat. 2. Some, who you would think are otherwise healthy, cannot fast (I have a friend that found out he cannot fast. He was instead hospitalized). 3. The severity of the fast depends on the time of year the fast falls and where in the world you are. Right now, it is certainly worse in Norway (where the days very long in June) than it is in the southern hemisphere, where the days have their winter lengths.

The last two can be somewhat accounted for (and already are: Folks that can't fast are exempt and special rules for places that get midnight sun). We can't really control for the first one, especially with real cultural taboo against breaking the fast during the day.