the 5S has those different coloured dual led flashes on the back, i think Apple calls it "true tone flash". in most other phones when you hear dual led flash it means that it's brighter but Apple went a different direction in aiming for more accurate rather than brighter. so what it's doing is trying to analyze the subject of the picture and produce a specific flash "colour/tone" that will give the best picture instead of washing out or reflecting. so the phone turns on the flash, analyzes then the real flash with proper tone kicks in and it takes the pic. so i'm guessing that the reason that it helps if you focus before you take pic is because then it already knows what type of flash to produce since you've told it what you want to focus on.
hope that helps. if i haven't explained it well, just google "true tone flash 5s" and i'm sure there's articles that do a better job than me.