back to the future - Why does Doc ask Marty "Never?" when Marty tells him George had never stood up to Biff?

In the first movie of the Back to the Future trilogy, when Marty comes back from the "Enchantment under the Sea" to get sent back to the future by Doc, he tells Doc that his father stood up for himself and punched Biff, and that he had never done this before.

To which Doc replies "Never?" while looking at the photo of the three McFly kids.

Marty: He laid out Biff in one punch. I never knew he had it in him. He never stood up to Biff in his life.

Doc: Never?

Marty: No, why, what's a matter?

Why is it hard for Doc to believe it? Is it because he looks at the picture, and sees Linda McFly looking a little shorter and chubbier than the two other kids, that he assumes Biff raped Lorraine?

This theory claims something somewhat similar, but I think maybe even the way things happen in the first film, that Biff might have had time to impregnate Lorraine.

Answer

Because if it never happened in Marty's experience, then Marty didn't completely fix the timeline.

Doc was envisioning what might have happened to the timeline now due to Marty's interference. He wouldn't know exactly, not knowing Marty as well as Doc would in 1985, but he'd know something was up. George would now be a different person, and make different choices, and that could thrown any number of variables out of whack.

After a moment's consideration though, since he's looking at the restored photo, I imagine his brain simply goes, "Close enough." Marty's not going to disappear again, and any future he gets back to he's just going to have to deal with. They have a lightning storm to catch, and nothing more can be done about 1985 with the time they have.