This page briefly introduces the sound pressure distribution caused by ceiling speakers.
Although this explanation is not physically rigorous, it is sufficient knowledge to understand the design guidelines.

Connecting the areas around the ceiling speaker with the same sound pressure creates a balloon-like surface. This is called an "isosonic pressure surface."

This "isosonic pressure surface" is difficult to call, so in this explanation we will refer to it as a "balloon."
The picture on the right shows a 70dBspl balloon.
(The picture on the left is viewed from above.)

What is the sound pressure inside the 70dBspl balloon?
It would be easy if it were uniformly 70dBspl, but unfortunately the inside is not the same 70dBspl. The closer you get to SP, the higher the sound pressure will be. So there's an 80dBspl balloon inside, and a 90dBspl balloon.

In other words, the balloon with low sound pressure is on the outside, and the balloon with high sound pressure is on the inside.

Consider changing the volume with AMP or attenuator.
The image on the left shows the volume turned down, and the image on the right shows the volume turned up.
For someone listening directly below the speaker, the situation shown on the left may not produce enough sound, while the situation shown on the right may be too loud.