I do not know the reason but I would love to know it; I use the 5s + s5 together every day, all day for business. And it is very simple; after about 5 hours the one I used most will be dead (it differs per day which one that is) and for the s5 the day will be over; it will take more too long to charge to even put in the loader, while if it's the 5s, it'll be fully in a jiffy. Now I could have a faulty model (I can rule out chargers as I have many of them, always use wall chargers and official cables) but I have the same experience with every other non-Apple phone I have. For tablets I have; iPads + Windows 8 tablets take the longest to charge, Android the shortest.

Anyway; it does not really matter; I like Android phones but for me short battery life (like you said, depends on many factors and is about similar on iPhones) life is killer and then long charging adds insult to injury.