Are all the years 'full years'? If you have had gaps between jobs, or worked term-time contracts, for example, you may have missed a couple of payments and the year won't count.
You can check your record on the Yougov site. You need to register, which is a bit of a faff, but once you've done that, it's easy to see everything in one place. I am not yet at spate pension age, so I can't see mine, but I'd be surprised if the way in which your amount has been arrived at is not spelled out on there - you can certainly see your NI contributions at a glance, as well as how many full years you have.