Woot! This old McCoy again!
As a rough guide....
Kill things slowly then the (one) drop from each monster goes into the loot allocation code and is evenly distributed between the party members in range.
Kill things to quickly and then one drop from each designated mob goes into the loot allocation code. Commonly known as the RoK/AoE nerf/bug - Anet have never confirmed if this is a deliberated nerf or just a really bad piece of coding that they can't/refuse to correct.
Bear in mind that a "mob" could be a single monster or any number of.
This is all easy to prove and has been demonstrated many, many times.