I think there is a difference between not feeling anxious and actually being calm. Even if you are slightly stressed your sac will probably be way higher than if you are in a really calm state. I am very conscious about my state of calmness (or lack thereof) just based on the timing of my breaths. You might try doing a count (1 Mississippi, 2 Mississippi, etc) to see how fast your breathing is when you first descend and start your dive, then measure again once you feel you have really calmed down during the dive and see what the difference is. Just to compare, when I'm zenned out, my breathing is about 7 counts in and 8 out (unless I am changing depth then I adjust the count to whatever I need - or if I see a shark approaching). My sac is usually about .40 on a normal warm water high visibility rec dive.
I can barely breathe that little laying in bed... maybe another kind of anxiety alltogether...

But I get it... and for some reason it just takes me quite a while to reallt calm down. The airintegrated DC is telling me the story pretty clearly. And then of course, thinking about using less air does not help at all.
I will try some BAT (big as tank) dives with my son on an AL80 and see if that in itself let's me calm down faster.