Whenever measuring in construction, it doesn't matter if the actual distance between two ticks is correct, as long as everyone uses the same measurement tool. amirite?

Let D be the distance from the counter surface to the floor, let h and w be the height and width of each can, respectively.
 
The first measurement will give a distance  D + h - w, the second will give  D + w - h.
 
Obviously,  D + h - w  cannot equal  D + w - h,  unless w = h.