Under the NEXT plans, your monthly per line cost is discounted, because AT&T separates out the built-in subsidy. So let's say your monthly cost would be $100 per month, then $15 per month per phone on the installment plan, which brings you back to $130. The difference is, after two years, that $15 per phone per month falls off and it's just $100 per month until you decide to upgrade, then you go back up to $130. The unlimited plan may not be eligible for NEXT, but that's in essence how NEXT can be cheaper.
I'll use my own situation as one more example:
5 lines. Old cost $290 per month with 10 GB of shared data, would get 16GB or 64GB iPhones for everyone, let's say 3 "free", one $99 phone and one $199 phone. Every two years that is approx $300 in new phone costs, plus the $290 per month (works out to $90 for the plan and $40 per line).
Under NEXT, it's $100 for 15GB, and $15 per line, totaling $175. Add in $15-20 per line per month for each iPhone installment plan, and we're right back around $290, but it's actually cheaper because that $300 in upfront costs is gone. After two years each phone is paid off and owned, therefore can be sold, and if we want to keep a phone for longer, then the plan is that $15-20 cheaper per month because the phone is paid off already. No interest, no hidden cost.