When using testfor to target a player via command blocks the block it ended up targeting was next to the one I had set.
I used 3 slight variations to test this
testfor @p[-724,53,-115,1]
testfor @p[-724,53,-115,2]
testfor @p[-724,53,-115,3]
the results of where I had to stand to produce successful output are displayed in the attached image

to reproduce
1:Select and mark a location to test. (if possible use X and Z coordinates that are negative, it may somehow be related to the problem)
2: Make a command block connected to a pulse signal so that it will check frequently wether or not you are in the right area.
3:Connect a comparator to the command block to transmit a redstone signal, from the command block, into some sort of signal for when you are in the right area. (I used a redstone lamp)
4:Turn on your redstone pulse and mark the areas that trigger a successful execution of the testfor command
5:this is where I saw that all of the different radius settings were all off by one block to the south-east. (my results are shown in the attached screen capture)