/testfor with rectangular search always returns true

When using a rectangular search for /testfor, this is always returning true.

Here's two cases of this occurring:
/testfor @p[x=351,y=61,z=264,dx=5,dy=41,dz=2]
/testfor @p[x=294,y=67,z=205,dx=1,dy=95,dz=0]

Radial searches are working fine, however.