Just been out and checked on my 2012 ES. I think there is only one average speed, and the trip A and trip B are the separate trip meters , but there is no separate A and B average speed display. I don't use the average speed as I have never found a need for it, but on my car it shows 35 mph whether I'm displaying trip A or trip B. Some functions can only be reset if total distance display is selected so that may be a clue, but as I don't want to reset my display I can't help much more. I use mpg on trip A which I reset regularly to check fuel consumption, and tend to leave B alone--except that it tracks overall fuel consumption since I have had the car , and my B mileage is the same as total mileage.
If you are worried about it why not pop back to the dealers and ask them , that's what I would do .
Sorry I can't be more helpful, but it's Friday and my brain hurts !