I am guessing that I needed 15.?? Before I could update to 16. Will update now will be after the update.
Don't be mad at me for this Honey Beagle

but I think the reason you are seeing it now? With 15.7.1 updating it took that 15.7.1 screen splash away. This would have moved the oddly smaller "update to 16" splash message much further up the screen.
I had this happen myself. "Hmmm, Why is 16 not showing, only 15.7.1". I had never seen this before (or hadn't noticed it ever). Apple was giving more than one upgrade version option. To get to the newer OS 16 upgrade option, you had to scroll all the way down beyond the bottom of the screen to see it. No scroll bar, you just had to flick the screen up (if that was not the case with you, my apologies

).