I have no idea what's going on with the years changing from 3 to 2 to 3 again, (you might have mis-read the years number as 3 earlier, though), but Tamas will sometimes evolve into adults when they're still 2. Ususally, it's because the Tama was paused a lot or downloaded at least once, since that seems to throw off the years. The number of years has nothing to do with how old your Tama actually is.