Nav Mesh incorrect margin

Hello everyone

I created a square plane mesh and I used it as walkable area to create a navmesh. I configured the agent radius to 0.5, so I expected the margin between the border of my mesh and the navmesh to be equal to 0.5. But this isn't the case. The margin is bigger and I don't understand why.

This is the agent configuration


And this is the navmesh (in blue) with the incorrect margin


I also tried with a regular plane mesh just to test and the result is even weirder. This time the vertical and horizontal margin aren't the same.

What is creating this margin? How can I modify it?
Thanks.