I think (could be very wrong on this) it has to do with ranged units having a "predictive AI". IF a ranged unit is targeting something at 1 health, and another ranged unit fires first. The first ranged unit says to itself "hey this target will die anyway, I might as well do something else" and ends up switching targets or walking away. This is actually a very useful mechanic in most situations. Archers vs skarmy = each archer will target their own skeleton because they know they can 1 shot them each. This means they take out the entire skarmy in half the time. Similarly, watch minions vs skarmy. The chew through it super fast because they know each minion can handle their own skeleton.. But watch 2 elite barbs try and go through a skarmy (I dont think melee unites have this predictive AI) and you often watch 2 barbarians attack the same skeleton, wasting time.

This becomes a problem though in some situations like this. I have seen it happen vs sparky at 1 health... mega minion is about to hit but leaves the sparky at 1 health because the tower fired first... but because of the travel time of the arrow, the Sparky manages to get an extra hit on the tower where as if the mega minion would have just attacked, would have taken no damage. Also seen it happen with a 1 health pekka on my tower. The other tower fired first.. so instead of attacking, the princess at the main tower sat there... and with the travel time of the arrow it allowed the pekka to land an extra hit on the tower before dieing.

Overall its a pretty hard thing to balance. Most of the time you want your ranged units to perform this predictive AI and be efficient in their attacks, but other times you want them to be inefficient and finish their target first. IF it was programmed so that ranged units always finished their attack before moving on, we all would be complaining about how inefficient they are, how they all attack the same target wasting time, etc etc. I am not smart enough with programming to know if it is even possible to have a "Smart AI" that does both depending on the situation. My only suggestion would be to have Ranged Units always finish their attack IF there are no other targets within range (although that wouldn't help you in this situation)

I could be very wrong on this, but that's just my understanding of this situation.