Thanks all for the replies. In case anyone else has the same confusion, I emailed tractor supply and got this response:
"Thank for purchasing our Black Diamond Medium Blasting Abrasives SKU#3905403. There must be a misprint on the packaging. We only sell two variations of this product: Fine and Medium. It should be the medium grit in the bag. If you are in any way dissatisfied with your purchase, you may return the product to the store at which you purchased it for a full refund."