US-84 doesn't have any exit numbers of it's own.
No - New Mexico mileposts US 84 as a north-south route with a zero point in Fort Sumner, since it's concurrent with other US highways east from there all the way to the Texas line (mostly US 60). So those are US 84-based exit numbers. If they were based on 285, they'd be in the low 300s.
Hmmm, I just put US-84 in the editor, flipped it, and striped out the US-60 segment and the mileage does then almost match up (3 miles off the exit numbers). Interesting for sure.
Adjusting thread title then.