Here is a good illustration.
These are the measurements of two 3" full range TB drivers. They are in separate boxes but the drivers are near the ends of the box. They are on top of each other keeping the acoustic centers to within 5" pf each other.
They are measured at 1 meter and with one watt. The mic is dead center between them.
Red and green lines are each driver playing individually, and the blue line is the two drivers playing together. Note too that it was not easy to get the mic dead center. Just a fraction of an inch up or down and cancellation would occur in the top end.
Now this is what happens when the mic is moved up 4". Note how the shorter wavelengths in the upper range are now out of phase.
Here is what happens moving the mic up another 4". It is now 8" above center. Note now that cancellation points are in two places.
Okay, up 4 more inches making the pic 12" above center. Now there are three cancellation points.
The change in vertical off axis hardly effects each individual driver, but you can see the curves change some from the mic being off axis to them. But the cancellation effects are tremendous. Dual tweeters stacked one over the other has the same effect.
This causes an irregular floor and ceiling reflection that takes away some of the airy spacial cues that help create a layered sound stage.