12" FINISHED is 12.5" UNFINISHED (unless the person who wrote the pattern incorrectly used the term).
Which block is it?
If you're all ending up with 12" UNFINISHED blocks, maybe there is something wrong with the pattern.
It means that it would be an 11.5 FINISHED block, which is really uncommon - it doesn't easily fit in a regulary gridded block pattern 3x3, 4x4, etc.
If you add up the FINISHED sizes of your block components from the pattern, what total measurement do you come up with?