How to even out the colour on wooden stair treads
On the risers (the vertical part between treads) you can see a faint shadow of the same pattern. It looks like maybe there was once a "stair runner"; a strip of carpet or other material that ran down the middle of the stairway. Sand more, try wood bleach (oxalic acid) as suggested, then use a darker stain to blend. Finish off with varnish.