all these equations are for the perfect world.
Have you asked for only one cubic yard of mulch, but the tractor has a three cubic yard bucket on it. the driver of the tractor scoops about 1/3 of a bucket full and dumps it on your trailer.
Or, the tractor has a single cubic yard bucket, and one employee fills that bucket to max capacity spilling mulch everywhere, and the next day a different employee uses the same tractor and bucket, but shakes and bounces that bucket, till you only get the bare minimum out of that bucket.
in all three instances you paid for only one cubic yard, but you get three different amounts of mulch.
I guess no one has any kidney shaped beds either, they are all perfect squares or rectangles? What about the circular beds with the tree in the center?
A good landscaper should know and/or have all the formula's necessary to complete a bid or project or test, but also there is some guesswork involved as well, once you land the job and actually have to get out there and do the work.
It sure will eat into that bottom line if you have to go back for one bag or one wheel barrow full of mulch to finish the job.
What works for me and my crew, may or may not work for you!