I can't comment to your physical location and how the units are installed, but you need to factor in PI vs. physical security. If you have a person that crawls in, that's a different scenario which I've never seen happen in an actual burglar event in a residence, same as a person walking in and waving their arms wildly.... 
You have to look at the patterns for the PI motions and what they cover and don't cover, but with the mask on, direct lookdown is disabled within 6' or so of the detector. Also, you need to pay attention to how a motion detects....across the pattern and not towards it.
Remember, a PI motion is designed to ignore a certain pulse count and pattern signature to approximately 2-3' AFF.
I can't say the DS835I's in my house and the blue lines I've put in mirror your test.
You have to look at the patterns for the PI motions and what they cover and don't cover, but with the mask on, direct lookdown is disabled within 6' or so of the detector. Also, you need to pay attention to how a motion detects....across the pattern and not towards it.
Remember, a PI motion is designed to ignore a certain pulse count and pattern signature to approximately 2-3' AFF.
I can't say the DS835I's in my house and the blue lines I've put in mirror your test.
