The problem is that catfish don’t got scales and can’t weigh their own selves.
Many home bathroom type scales are quite accurate and will indeed reflect a fairly close actual weight differential. The problem is the PERCENTAGE OF ERROR factor. A 10% margin of error on a 10 pound fish would only put you one pound off. But a 10 % error on 200 pounds would be much greater. On most home scales you can weigh the same package 3 different times and get three different readings.
That’s why it requires “certified” scales to qualify for any kind of weight record.