If you've made a block that's supposed to be 12 1/2" and yours came out 12" more than likely it has a lot of angle seams. Here's how you can fix it:
1. You can take it apart and sew it back together with
a "scant" 1/4" seam. or.....
2. You can trim all your blocks to that size. or......
3. You can trim it down a little more to 11 1/2" and
add a 1" border all around it.
If it's a pattern, make a sample one out of scraps and see if it doesn't come out to 12 1/2". Chances are it's saying 12" because that's the finished (in quilt) size.
Hope this helps