If you run your phone with the display off, say from 21% battery down to 11%, then it will cross the 20% reporting threshold. The next time that you unlock the phone and activate the display, you will get the notification.

It's not inconsistent; nor is it just being "optimistic" -- it may be poorly worded, but it consistently displays that same message every time you unlock the phone after the battery drains to 20%.

If the person who took that screenshot had waited another ten minutes, then the battery would probably have reached 10%, and the 10% notification would have been shown instead.