Pretty simple but annoying bug. Marker ArmorStands are not within r=0 of themselfs when they stand on the ground.

Steps to reproduce:

• Stay on the ground
• /summon ArmorStand ~ ~ ~ {Marker:1b,NoGravity:1b}
• /execute @e[type=ArmorStand,c=1] ~ ~ ~ /testfor @e[r=0]

This will find the player (if he didn't move), but not the ArmorStand itself.
If you do /tp @e[type=ArmorStand,c=1] ~ ~0.0001 ~ it can find itself again, because it no longer has a y-Position without a decimal place.
If you fly in the air and use the same commands, it also finds itself as long as it's position has decimal places.

From testing it seems like x and z Coordinates are affected too, but with the y Coordinate it can be reproduced more easily.