They had to make the magazine bigger for all the ads they needed to include.

I think it's a magazine with a special follower, which basically blocks out most of it and only leaves enough space for 9mm rounds. The reason to do this is so you can reuse a standard AR-15 lower receiver - otherwise, you would either need a whole new receiver design, or a magazine adapter (which is a similar idea to the follower, but applied at the magazine well rather than the magazine itself).

Parts reuse like this seems to be pretty common in the civilian AR market - there's a bunch of guns which are sold as separate upper receivers, so you can buy just that part and install it onto your existing lower.