I do the weighing at a sale barn. Our scales weigh in 5# increments - you may see a number ending in something other than 5 or 0 as an average on a draft, but the total weight for each recorded weigh will always end in 5 or 0. Sometimes it even amazes me how often even numbers like 400 or 500 come up, so keep in mind that, however unlikely it may seem, it is entirely possible that those were the actual weights on your calves. I can't imagine scales that weigh in any more than 10# increments being allowed to be used in a sale barn.
As others have said, the scales have to be certified twice a year. P&S (that's packers and stockyards, the federal regulatory group) requires that I record a zero balance at least every 15 weighs, and after every draft that weighs 10,000# or more. Even though I don't record it, if I don't see a zero balance come up on my monitor every time the scale is empty, I will stop and zero the scales. It's unusual to have to do that except when it's really muddy, though.
Also, once in awhile, a P&S agent will be setting in the stands watching the sale. After the sale they will go in the office and pull 10 scale tickets, and have those cattle brought back in and re-weighed.
Our scales are electronic, as are every other sale barn I know of. As an operator, all I do is push a button to tell it to weigh once the gates are shut, and punch in the number of head. There's really no room for hanky-panky, even if I wanted to. And plenty of oversight to discourage it as well.