c(amount) argument does not work in testfor

What was expected to happen:

The command block will only power the comparator when there is only 1 person in the area.

What happened:

The Comparator turned on with more than 1 person

Command: testfor @a[x=-800,y=11,z=4,dx=26,dy=12,dz=26,c=1]

notice has the c DOES equal 1 yes, yes it does indeed. Therfore this has been declared a malacious bug ruining many works of command blocks