When census information is collected they recorded everyone in the house, even if they were just visiting. So it could be that the Fitch grandchildren were just there for a visit?
Records can have errors but the marriage registration information was collected personally from Frank, so I would consider it the most reliable at this point. Unless a birth registration can be found, at which point you update your info based on that.
If Jacob and Jane are non biological parents then in the information boxes above the bio tick that off. You could explain your suspicions, that Frank was possibly the illegitimate child of Catherine Matthews and William Fitch but is thought to have been raised by his grandparents. It would be best to lay out how/why you know this in a Research Notes section. Finding a birth registration or death registration that gives his mother as Catherine Matthew would help support this.