Precuts can vary in size depending on the brand. It happens most often when precuts have pinked edges. Some precuts are measured from pinked edge to pinked edge while others do not include the pinked edges. Before you begin sewing, measure your precut blocks and trim if necessary to make sure they are all the same size. No matter what size the precut happens to be, if the pieces are all uniform, you'll have success. It's a good idea to follow the same measurement when you are sewing as well. Using a 1/4" foot on your machine will keep your seam allowance consistent and as Jenny says, "as long as you are consistent, your block will turn out!"
In the rare occurrence your pre-cuts are extremely mis- cut please take pictures and email them to firstname.lastname@example.org. We share these photos to be better for you.