Chris G at Airbnb just advised me that there are TWO places where 'last updated' dates can be found and that they can show different dates - because there are two different definitions of 'last updated', depending on where you are looking.
He said, "When a guest is looking at your calendar and selects the check-in or check-out dates, there is a message at the bottom of the calendar that says when it was last updated... that "update" is referring to the last time you viewed or edited your listing description. Anytime you go to the Manage Listing page of a specific listing, it will change that to say "updated today." Just by viewing that page, it will be "updated."
This tool is in place to help guests know that their hosts have an up to date calendar and haven't been away from their computer for long periods of times.
When I explained that I was referring to the last updated date under the heading 'Your Listings', which showed a different date from what the guests see, he then said:
"I can certainly understand the confusion when they both say different dates. This is because one is guest-facing and one is host-facing. As we discussed, the "last updated" message appears on your listing to guests to let them know that the host is an active user and hasn't been away from their computer for long period of times. It updates anytime a host views the calendar.
The "last updated" message under Manage Listings refers to the last time you made changes to that listing. If you added a comment to the listing description, changed your Instant Book settings, or updated your pricing on a specific listing, it would "update."
I found his answer to be very helpful - I hope it helps you too.