When I use the command — testfor @e[x=~,y=~,z=~,dx=0,dy=1,dz=0] — there is an extra 1 block distance detection in positive coordinates. Another example — testfor @e[x=~-1,y=~,z=~-1,dx=2,dy=1,dz=2] , it is a 3x3 area but because of the bug , it is 4x4.

Further explanation — base on the previous example , it should be only 3x3 , but because of the bug , it is +1 block on the positive coordinates which is +x and +z , it became 4x4.