[PvK]

I see the same thing, even over open ground, so it's not about obstacles.

2 diagonal + 1 orthogonal is 3 x 2 away, or square root of (3x3 + 2x2 = 13), so that is over range 3, and makes sense it is out of range.

1 diagonal + 2 orthoganal is 1 x 3 away, or square root of (3x3 + 1x1 = 10), so that's just a hair over absolute range 3 (square root of 9), so it seems the game uses actual distance according to the Pythagorean formula, but with a slight nudge.