Why do we use the distance to the centroid when finding the resultant force?

[aero&astro]

When find the resultant force on a plane surface under uniform pressure why do we use ybar, the distance to the centroid when the resultant force acts through the centre of pressure?

Shouldn't we use the distance to the centre of pressure?

 

[AlephZero]

If the pressure is uniform, the center of pressure is at the centroid.

 

[aero&astro]

there's always two different values for the distance to the centre of pressure and to the centroid though?

 

[AlephZero]

For the second time, in your question the pressure is uniform.