In the actual deposition, it looks like the expert actually does some calculation with the calculator. And I imagine he did the math and saw it was different from what he had stated.
The lawyer deposing him isn't an "expert" and isn't allowed to introduce evidence by himself (even if it is patently obvious). The only tool he has is to ask the expert in front of him. If the expert refuses to comply and show himself to be wrong, the lawyer has no recourse but to bring his own expert to court to refute him.
I don't know what went wrong here (measurements were off, conversion was off, drawing wasn't to scale, whole thing is wrong), but if the expert admits something isn't right, it would presumably allow the whole thing to be excluded. The expert is trying to recover and cover his ass here. He knows something isn't right, but probably knows that 68 feet is the right answer. Rather than sink his whole ship, he's trying to focus on the actual accident.
I think his testimony is worthless based on his mistake and the deposing lawyer was right to push the issue.
I am not a lawyer.