Either wooden planks or the stairs and slabs have their bottom texture reversed--the lines that separate the apparent 4 planks don't line up.
To duplicate, place a wood plank of any color (tested it with oak and dark oak, assumed to effect all planks) DIRECTLY in-between a stair and slab of the same color, then destroy the blocks underneath and look at the texture.
Two pictures are provided to save the trouble.
Oops sorry, I thought I checked all the duplicate-potentials 😛