Both sides are the same piece of plastic, so the card must end up the same size in both boxes. The red and blue outlines mark where printed content starts in each crop — if they don't sit on each other, your two crops are at different scales and the print won't line up. Match back to front fixes it in one go; the size slider and nudge are for the last fraction of a millimetre.

The outline follows ink, not plastic — a card that is white right up to its border reads as slightly inset. Treat it as a measurement and trust your eyes on the real edge.