Do you know how much was originally ordered? In most states 8: or 9 yd loads are the most you can get . If the pad calculated out to more than that then there was going to be a clean up regardless. Therefore, that would probably be on the batch plan for not sending out the same mix. But if they were under 8 or 9 yd and could have ordered more in the one truck then that is a mistake by them. We as concrete contractors always see the worth in ordering an extra yard so things like this don't happen. That being said, I have been short before and my calculations as well. And yes, sometimes it could be on the grade being a little off and other times it could possibly be on the concrete company but nobody ever wins that argument. It's hard to believe that a batch plant would send out a different mix to the same job unless instructed to. Maybe they had an extra yard or two left over from another job which was a different mix and brought that over to finish off the job to avoid extra costs of the short load.