2 shops, different alignment #s?

Wife's Xterra was about due for tires when I noticed the outer tread block on the RF tire completely gone. The other tires were pretty evenly worn. Took it in for an alignment at a national chain but they said it was alright. Printout shows everything is green. They said all four tires were a bit low so that probably caused it. The Xterra has TPMS. It's pretty aggressive and not easy to ignore when it thinks the tires are low so I was a little dubious of their claim that all four tires were down to 20#. But, if they were ALL low maybe it doesn't go off. I'm not sure how that works on this car. But, how would all four tires being low cause ONE of them to wear funny.

Got some new tires at another shop that only does tires.
A couple weeks go by and I can't shake the thought that something is wrong with this picture. I certainly don't want to be ruining a new tire. Today I'm off so I took it to a regional chain store to get a second opinion. THEIR printout is pretty close on all the numbers but, shockingly, the toe is out.

Question: I know there are differences in measurement machines and they probably need to be calibrated at some interval but how much variance is normal for alignment racks? How much of this could be down to the mechanic?

Any play in your suspension components? like balljoints? Could be why the numbers dont match even though you didnt have it aligned.

When my chevy van's balljoints were shot, the outside edges of my tires were getting murdered. By the time I noticed, there was a lot of play. And of course I noticed after I inspected and re-packed the wheel bearings. Doh!

If theres excessive toe out, I would think it would cause more wear on the inner edge of the tire, not the outer edge.

Meant to say that the toe was outside the range. Good point though. I'll check the joints.

Some shops don't "settle" the car after jacking things up or driving on the ramp so ride heights can stay high bumper steer from the factory can effect the toe setting.

Anything with shocks on it should get a push on the bummpers after any jacking is finished

I like to roll it back and then forward again after every adjustment to take the flex out of the tires when I'm doing alignments on the race cars.

if all four tires were low the tpms light would be on also.
From my experience it seems like Nissan tpms lights like to cut on if any tire is below 26 or so psi.

regardless, if all four were way low for a long time, it should have worn all four on both the outside and inside edges. high pressure wears in the middle and leaves the outside and inside alone.
Front Toe being out is to be expected from any car with any sort of mileage racked up on it, every car I've ever done an "alignment check" on has had toe in the front at least slightly out of spec