The amount of food a seal eats, when you weigh it, depends very much on the energy content. So if you're eating a very fat, rich fish, you consume less of it than if you're eating something that doesn't have a whole lot of calories in it. But yes, that's in the correct order of magnitude, but it can vary by a factor of three.
The other point I would make is that just estimating how much an animal eats tells you nothing about the potential impact on fisheries. The way we tend to think about things is to have this simple view of the world in which seals eat fish, so obviously more seals mean fewer fish. But the marine environment's more complicated than that, so let's just make it a little bit more complicated and put in a three-step system whereby seals eat the predator of a commercially important fish. When you have that situation, fewer seals mean fewer fish for fishermen. I think that's the complication that people tend not to understand, and that's one of the real problems in explaining to people just how complicated marine systems really are.