What’s with all these "Affordable" units at apartments?

I'm looking at a unit that's listed as "affordable housing" but the lease is 13 months at $1971 per month, not including any additional fees. The qualifier for this unit is that you need to make less than $50k in gross income. That's more than half your after tax income just on rent, not even including utilities. I've been seeing more of these units everywhere, and I don't understand how this is even legal?

This same complex has had the same 10 "affordable" units available for almost a year now, so they're obviously not affordable at all.