Everything has a cap by tier. See chart below for maximums.
If you see items higher than the cap, 99.9999% of the time they are 'unspawns' from before the caps were put in place. How ever, there is fable of overspawns that break the caps. But I haven't seen or heard of these in a long time.
http://i.imgur.com/Le9JtXC.png