Pencil tool in Ai - pixels problem (Unit of measurement in grid and pencil tool is different)

Pixels in the grid and pixels in the Pencil (Pencil tool) — different measurement results are obtained.
If in Illustrator you do Edit – Settings – Units: General – pixels, Stroke – pixels.
And also guides and grid: grid line every 10 pixels, divisions of 10. It turns out that each small square is one pixel.
In work Document setup - also pixels.

Draw a square 20 by 20 pixels, select it.

Then select the Pencil tool, in the Pencil tool options check ON a "Edit selected paths", specifying within 20 pixels.

Try to bring the Pencil tool at a distance of 20 pixels, or even 10 pixels - nothing will happen. But if it's very close to a square, then the Pencil tool will work (the asterisk disappears from the icons) and you can change the shape of the square. It feels like within 20 pixels these are some other pixels.

And if you set it within 5 pixels, then the Pencil tool will look even closer to a square.

Why doesn't the grid match? Or is there something in the options inside - do it mean other pixels?