The CommandBlock's Radius checking seems to follow into the pattern of the South-West rule, I was testing some ideas such as a "Respawn Countdown" idea and found this oddity.

I have 2 command blocks set up in a 16-tick loop with the following commands;
/scoreboard players remove @a[r=3,x=355,y=64,z=-180,score_Countdown=10,score_Countdown_min=0] Countdown 1

/scoreboard players set @a[r=3,x=355,y=64,z=-180,score_Countdown=0] Countdown 10

I found that depending on where I was stood within the area, I would not decrease in my count-down. I investigated into the Diamond shape radius check but have found that it's not fully diamond...

In the provided Screenshot, if my feet were at the RED location. my "Countdown" would not decrease.
If I stood in the GREEN location. My "Countdown" will. The green is FURTHER than the red, and the centre is not changed between loops.
The white/grey wools are a radius guide, inbetween the DARK grey is the "Black" which is the origin of the radius check.

For the record, NEITHER location SHOULD decrease the "Countdown"