It could be either or both, but I think it’s the PIR reacting under changing conditions. I have a front porch camera that picks up vehicles coming up the driveway better than going out. I think that the heat signature is more defined on a vehicle with a warm engine. I also get different results with day vs. night…sometimes!
PIR detection is not an exact science. So many factors in play here. Detection works best when the object is moving perpendicular to the sensor, and is “warmer” than the surrounding area, which makes it stand out.
Play with sensitivity. Play with camera angle. If you’re concerned with WiFi strength, consider moving the camera or adding a repeater. Good luck, and keep us posted.