Eagle Cast cranks - QA?

Apparently some are 10th out and some 15th - so to achieve a correct index they all have to be taken down 30th - does that sound right?

I understand indexing brings the rod throw to an even height but lIke I say, Im not really clued in to the whys and wherefores - just that it needs indexing.

Im just wondering if this is "par for the course" with Eagles?,

and also if its a blue printing process that people dont do when they build a trailer motor?