Is armoured/metallic BX cable required for exposed wire in basement ceiling?
They have more flexibility in how the AC is installed than if they'd used NM.
For example, in most areas, the NM-B cable you have stapled to the bottom of your joists is not allowed; it should be running through the joists like the other one.  I wouldn't suggest changing it; it just doesn't meet today's standard.
But if that NM-B on the bottom of the joists were AC, it would pass an inspection.
The HVAC contractor would probably rather carry one box of AC than several different materials to minimize material costs but taking up more room in their van, or causing more trips to the supply house and running up miles &amp; hours.