The case I have is of a husband and wife who have 1 child. The husband then died and the mother had a second, but illegitimate child some 4 years later. No father is named on the illegitimate child's birth cert, but when he later moved away and married, he recorded his mother's deceased husband as his own father on the marriage cert.
I presumed this was to hide the fact that he was illegitimate to his new wife's family. I did wonder though whether he might have grown up believing he had the same father as his brother, the passing of time being rather less well-measured in those days. However that would have also depended on the restraint of village gossip, so probably unlikely! I would love to know the circumstances.