Looking further at the Banns book, rev John Dickinson really is confused!
At the beginning of the page there's a set of banns in Oct 1756
Then 3rd, 10th, 17th April 1757, then your couple on 5th, 12, 19th Dec 1757, then a couple on 1st, 8th and 15th May 1757, then Sept / Oct 1757.
The 5th Dec 1757 was a Monday not a Sunday, the 5th Dec 1756 was a Sunday. I think Rev Dickinson forgot to write the Banns in the book when he should have done and only remembered when the couple got married a few months later, then wrote in the current year not 1756 as it should have been