To reproduce: Make a world, make a command block clock, in the command block type:

/execute @e[type=ArmorStand] ~ ~ ~ /spreadplayers ~ ~ 0 1 false @e[r=0,c=1]

Then summon an ArmorStand, watch it go in negative coordinates.
For easier reproduction open 1.9, make a void world, place a repeat command block with the above command and summon an armour stand, it will favour the negative x/z corner of the platform.

EDIT: Added video evidence, video includes F3 menu for computer specifics
https://youtu.be/qoL3O9j_zjY