"Adjust" may not be exactly the right word.
"Accommodate" may be a better choice.
The point is that it's not possible to make two pairs of holes, e.g. in stringer and tread, line up perfectly so that bolts just slip in easily. Making one of the four holes 'obround', allows for a practical amount of uncertainty of location.
Mike Halloran
Pembroke Pines, FL, USA