When the player runs on pressure plate above TNT, the coordinate calculation may run into problem

When players run on the pressure plate above TNT, the Y axis calculation may run into problem, for example, I placed a TNT platform on y=-60, then place some pressure plate on the TNT, and then I placed some command blocks to test that where am I, here's the command:

/testfor @a[x=70,y=-64,z=-35,dx=20,dy=1,dz=20]

by principle, it should test that I'm on the axis range y=-64~-63, but when I'm running on the platform, it successfully tested me who's on y=-60(in other words, not fell from the platform), and it shouldn't happen(because I'm out of that range).

How to reproduce:

1.Create a superflat world, and find an area and fills it into air:

/fill 70 -64 -35 90 -60 -15 air

2.Give yourself a command block and a repeating command block, and place it like the figure 1, and fill command 1 in the repeating command block (as above actually); fill command 2 in the command block:

COMMAND 1: /testfor @a[x=70,y=-64,z=-35,dx=20,dy=1,dz=20]

COMMAND 2: /say success

3.Launch the repeating command block by Block of Redstone and create a TNT platform like this:

COMMAND 3: /fill 70 -60 -35 90 -60 -15 tnt

COMMAND 4: /fill 70 -59 -35 90 -59 -15 acacia_pressure_plate

4.Run on the platform but have to notice one thing that don't fell from the platform.

Observed result:

You will find that "success" appear on the chat sometime.

Expected result:

It shouldn't happen because the player is always on y=-60, but the command tests who's on the y=-63~-64, and it miraculously executed successfully.