Agree ... if you are getting a "block" that is supposed to "finish" at 12" ... when you are done making the single block and ready to put it into a quilt that block should be 12.5".
For example to use a commonly used source - when you look at QuiltersCache and she indicates "this is a 12" block" - that BLOCK should be 12.5" when you are done making it.
I too have had problems with some of the free blocks on some sites so I have taken to either measuring the individual cuts in the pattern, or making the block in EQ7 to get the right measurements.