On attics, even with lots of insulation up there, it's air leaking out through gaps in your vapour barrier that can undermine your insulation. Regardless of building codes or builder marketing claims, your levels of insulation may be surprisingly meaningless if you are leaking air up into your attic apace. Just reading on this now, is this blower door test a part of any test performed prior to delivery? Or do the builders just get to say they are building better, tighter? Does not appear to be a part of any PDI in this neck of the woods anyway. Seems that's the only way to prove anything.
Speaking of plumbing, on the excursions into my attic to prepare for an insulation top-up (which, full disclosure, has yet to occur), I found surprisingly large gaps around plumbing vent penetrations, round pipes passing through square holes/gaps at the tops of wall headers, with scraps of batt insulation packed in, with a bit of red VB tape loosely applied. Inadequate. Also found needlessly large drilled holes for electrical wires in the tops of wall headers for wires feeding ceiling fixtures. In a few areas, the guy seemingly only had a 3/4" drill bit on hand for holes to feed single lines of 14/2, making holes about twice as large as needed, making huge air gaps around a few wires. Pathetic. What all of this does is to create a chimney effect, warm air rising to the attic in every spot. A couple cans of spray foam emptied out fixed all of this, I'm calling it good for air tightneess up there now.
I wouldn't think your "furnace differences" could be a variable. The thing is either safely making heat and the blower is blowing or it's not. Since I'm onto builder deficiencies, what about HVAC supply line assembly? If there are untaped gaps in supply lines inside of wall cavities, you're losing heated air to needlessly heat the inside of a wall, or to leak into the attic or garage. I found a few of the visible connections on our basement ceiling untaped when we moved in, I taped off all I could get at, leaving me presuming that there are imperfect connections inside of every wall, building codes notwithstanding.
Are these rentals Carl? I sure hope heat loss is your biggest concern with these items going forward! As you can see, it's a priority with our primary residence!