The plates probably have some very minor differences hence the markings. Regardless, if one pedal is more difficult to clip into and both are set to the minimum tension, check your shoes to make sure the rubber sole isn't interfering with the pedal. If it is, you can either center the cleat more or if that's not possible, use a knife to trim away any excess rubber. Also try lightly oiling the springs and see if that helps.
If you can live with the higher tensioned pedal, then the other option is adjusting the easier one to match. It could just be a manufacturing tolerance issue stemming from the torsion spring.