Spreadplayers not consistently working

When running the same spreadplayers command under the same conditions, it will only succeed some of the time.

Steps to replicate:
1. Summon 12 chickens (the number of chickens is important as it seems correlated with the bug): /summon Chicken -7 56 6 {Invulnerable:1,Attributes:[

{Name:generic.movementSpeed,Base:0}

]}

2. Set up a spreadplayers commandblock with a spread distance of 2 and a max radius of 3: /spreadplayers -20 -9 2 3 false @e[type=Chicken]

3. Trigger the command block numerous times. It seems to work about 1 in 5 times. Shouldn't it work every single time?