dx,dy,and dz are very inconsistent. for examply, if an armorstand has tag delete, and you run
/execute @e[tag=delete] ~-0.3 ~-1.8 ~-0.3 /testfor @a[dx=0.6,dy=1.8,dz=0.6], it only targets players more than 1.8 blocks down (not less than, that is within 1.8 blocks down), as if the dy extends down rather than up. it also targets players up to 0.6 blocks in the negative x and z directions, and no distance in the +x and z directions, as if the sign on dx and dz was reversed and the offset was 0. when the offset is +0.3 and +0.3 instead, it targets players up to 0.6 blocks in the positive directions, but 0 in the negative, as if the offset didn't exist.
there is probably many other buggy behaviors that I have missed here also.

to reproduce: summon an armor stand with tag delete
run execute @e[tag=delete] ~-0.3 ~-1.8 ~-0.3 /testfor @a[dx=0.6,dy=1.8,dz=0.6] in a repeating command block with a comparator
see that it turns on and off at weird places