That is the new "Only 10 Priority Rewards" bookings allow per member started this year.
The said they were going to add this feature to the dashboard last week (or the week before), I guess AC IT finally get that to work.... almost.. as you stated, when you click on the ? it state:
This chart shows your Altitude Qualifying Dollars (AQD) earned since January 1 on eligible flights. AQD represent the monetary value associated with an eligible ticket and, when combined with the number of Altitude Qualifying Miles (AQM) or Altitude Qualifying Segments (AQS) you fly, will determine your Altitude status.