2018 Mini Multiple Monitors - What Determines Monitor Number?

[DFWHD]

I just moved from a single monitor on my 2018 mini running Big Sur 11.6.4 to a dual monitor setup via USB-C (2 Dell U2722DE monitors). The monitors were chosen primarily for my work from home setup with my Dell work laptop, but added bonus is I benefit from using them on my Mini. They work great, but one thing puzzles me. I've used the display preferences arrangement tab to set my left monitor as primary by dragging the menu bar to the left monitor and the left monitor shows as Dell U2722 (2) and the right monitor as Dell U2722DE (1). Nothing seems to change this, including reboots and swapping the cable positions/ports on the mini. Anyone ever seen this? like I said, monitors work great, just would really like to know what drives that number...