On a curved hem, like on most Iron Heart shirts, the length of seam allowance edge (brown) is longer than the length of the seam stitch (orange). This creates a tendency for the fabric to curl towards the longer side.

And, there are 3 thicknesses of fabric on the hem.
Given the thickness of the resulting seam, it is easier for the fabric to roll away from the bulky seam, than to roll towards the bulk.
It is something that can't be avoided, and the tendency to roll up is a direct function of the bulk and weight of the fabric. The heavier/bulkier the fabric and thus the seam, the more pronounced the roll outwards will be...
