Lightmapping: How does it actually work?

I can't make sense of what's happening. I'm having one emission box and one floor. The size of the lightmap alters the baking in unexpected ways.

The emission cube is at the position of the axis in the top image.
I have then changed only the lightmap scale for the ground object.I've turned off the gaussian filter so it is easier to see the baking.
Image 1: 0.2
Image 2: 0.19
Image 3: 0.11
Image 4: 0.1

Why are the results so different? In the last one, I get no light baked at all.
Unity 2019.4, URP 7.5.3

 

You probably have too low indirect samples to have any consistency in the bakes.

 

That's what I thought at first, so I increased the samples to 5000. Made no difference at all.
Well, I thought. Maybe I accidentally increased direct samples instead of indirect. I can't replicate it now.
You are probably right... thanks!