Mounting camera upside down solved motion detection issue - why?

[Teuton]

I mounted my new Arlo camera rightside-up about 7 feet above my back porch angled about 40 degrees down and it would not reliably detect me walking across the field of view about 5 feet away unless the sensitivity was set to 100%.  At 95% it occasionally detected me, and at 90% it didn't detect me at all.  Unfortunately, 100% sensitivty outside results in a steady stream of false alerts.  After reading some other posts here, I decided to try mounting it upside-down and, to my surprise, it now reliably detects me with the sensitivity set to 75% (which means virtually no false alerts.)  I'm a techie, but I can't for the life of me understand why being upside-down made such a difference.  Anyone have any ideas?

[TomMac]

it could be the difference in the Fresnel lens for the PIR... maybe "weighted" more on the top ( which is now on the bottom )