I'm going to add a couple things to what Macybaby said.
1) There are a few documented cases of mistakes in the Singer lists. If you have carefully written down your serial number and looked it up properly(using the right number of digits), checked it three hundred times, had someone else check it a hundred times, and it still says your machine is a 27 when it's obviously a 15(just an example) then you've found one of those mistakes.
2)When Singer started that certificate thing, for any machines they didn't or still don't have an accurate record(such as the early 301s) they chose an arbitrary date for all machines of a given prefix. For example, for all of the NA prefix 301s, Singer gives everyone the same manufacture date for their machine. Same goes for the NB prefix machines and several others.
Cari