According to the extended analysis comparing all three generations, Robert was not Dave’s biological father.

When the Table Stopped Breathing
The words settled into the room like something irreversible.
Patricia went pale.
She said it was absurd. That those tests could not prove anything.
Robert looked at her with a steadiness that was harder to face than anger.

He asked her how long she had known.
She did not answer.
He asked again, the same question, the same quiet tone.
Her lips trembled.
Robert told her that he had carried suspicions for years. That he had chosen not to look too closely at them.

Dave was staring at his mother now.
He asked her slowly and directly whether it was true.
The room waited.
Finally Patricia said, barely above a whisper, that it was a long time ago.

Dave pushed his chair back from the table.
He said that she had spent five years accusing his wife of exactly what she herself had done.
He gestured toward Sam, who was still focused on his dinosaur drawing without any idea of what was unfolding around him.

Patricia looked like the chair was the only thing holding her upright.
Robert stood slowly.
He said, with a steadiness that I have thought about many times since, that he supposed that explained quite a lot.

Then he looked at me.
He apologized.
He said I should never have been treated the way I had been treated in his home and at his table over all those years.
I told him thank you and meant it completely.
Then Robert turned toward Sam and said that regardless of what any piece of paper said, that boy was family.