Good for you for being so consistent. Those tests don’t lie and it seems quite likely that your AeT has moved up substantially if you’ve been following the training. Six months is probably as often as you need to consider doing these tests. There is another way to monitor AeT improvement that I will explain in more detail in a soon to be article. If you have a Training Peaks Premium account there is a metric shown as Pa:Hr. This is the ratio of your Pace to heart rate as measured in the first and second halves of a workout. If the ratio exceeds 5% then that a=workout was conducted above your AeT. There are a few restrictions that need to be controlled for but it is a handy tool that can be used each and every workout.
Watch for this article in the next week or so.