Square brush size incorrect

the square brush size is wrong image in this image i have a 2 pix brush againt a 3 pix as you can see this brush is not 3 pix but 4 pix which shouldnt be happening

The brush sizes are 1x1, 2x2, 3x3, 4x4 etc.

I can’t seem to reproduce this behavior. Selecting square brush and size 1, placing it down, increasing the brush size by exactly one (by using numpad +) and placing again, I got this result, the squares consistently go up by exactly one pixel.
Are you sure you had 3px and not 4px selected? Try switching it with the numpad+ button. Also, which version of Aseprite do you use? Did you use any other settings that might have affected the brush size like pressure sensitivity?