Let's face it, sometimes information on a document is flat out incorrect. It may be that the ancestor outright lied, someone misunderstood something, etc. but the fact remains. One document can be completely wrong on one item. It happens.
When you think you have a situation like this, organize all your documents and outline your reasons for why you think the one document is wrong. That will help you make your case and allow others to see if they agree with you or not?
And once in a blue moon, I think someone just gave a "funky" answer to a question on a record just to be clever. And that's what confuses some of us today.