Ok, I think I may have figured out what may have happened: admin error!
I had a localhost dev version of the live production site running, managed users locally, and I’m assuming the plugin still was communicating with MailChimp even in a local/dev server environment.
So many dif. settings, I had forgotten about that syncing functionality and didn’t figure it was still syncing from localhost.
Human error. Lesson learned.
If this sounds like it may NOT be the cause after all, please chime in – but this is my best guess.
Thanks.
Also, if this is helpful info: 100% of the 210+ or so subscribers subscribed via the Autochimp WordPress syncing process:
1. became members of the site (using S2Member membership plugin)
2. Autochimp syncs them to MailChimp list.
There is no other direct subscribe form promoted.
The full 210 or so members are still users inside WordPress.
Thanks.