Why does that matter? Just because one says something, doesn’t mean it changes anything about how something really is. It means the other is either incorrect, or is using a different reference or definition. I don’t see how that can be considered a sin.
To deny an eternal truth -- if you know it to be one -- whether to yourself or to others, is to miss the mark.
Now if you don't believe in eternal truths, you may still call a green leaf gray.
That merely makes you a common liar.