If an ArmorStand has Marker:1b set, it cannot be found with r=0.

Setup:
Stand somewhere, without moving
/summon ArmorStand ~ ~ ~

{Marker:1b}

/testfor @e[r=0]

What should happen:
Found

{player}
Found Armor Stand

What actually happens:
Found {player}

This is not because of the relative position of the stand; If you walk a bit away and run
/execute @e[type=ArmorStand,c=1] ~ ~ ~ testfor @e[r=0]
the command will fail.

Other entities may be selected within the range, however; If the first experiment is run with a NoAI Sheep summoned as well, at exactly the same position, the command will find the player and the sheep, but not the stand.

Notes:

• A range of 1 or more results in correct behavior of commands.
• Non-Marker stands can be selected with r=0
• A possible cause of this may be the way entities in range are checked in the code; As the Marker ArmorStand has no hitbox, selection by range may be inherently buggy due to the calculation method.