I'm using Physics2D.BoxCast in Unity(2019.4.19f1).

I have a collider2D at position==(0,0) with scale==(1,1)
Then I call a Physics2D.BoxCast:

var hit = Physics2D.BoxCast(new Vector2(0, 2), new Vector2(1, 1), 0, new Vector2(0, -1));

It is a Box Ray Cast like the following pic:

I have a hit returned.
But hit.distance == 0.9950001, which is supposed to be 1.
I guess there may be some inaccuracy in Physics2D.BoxCast. But I have no idea how much the inaccuracy can be.

Then I do some searching and find "Default Contact Offset" may be related to my problem.
I change "Project Setting → Physics2D → Default Contact Offset" to 0.001(default is 0.01).
Run the code again. But now I get hit.collider == null.

Now I'm more confused about Physics2D.BoxCast.
It may be helpful to learn something about how Physics2D.BoxCast work. But I do not know where to find them.

Added 2021.3.1
I did several experiments about "DefaultContactOffset".(link)
The experiment results show that:

1. If the DefaultContactOffset is about 0.002~0.01. Most of the time, the error in hit.distance is about 0.005.
2. If the DefaultContactOffset is about 0.02~0.1. The error in hit.distance is larger.