UNSC are made of secrets. Which makes me think these are Second generation SII´s. Mendez leaves the original SII´s to train the second generation. The project get closed down(as mentioned in one of the books), because of the costs, and not enough candidates. Maybe UNSC kept it going. That or they are just a team of Spartan II and III´s. I doubt the main character is a part of the gray team. But who knows.