I think of a cat's tail positions as akin to a human facial expression. A smile is an indication of happiness, but how an individual person uses it to express their mood is highly dependent on the person. One person can be boisterous and quick to smile, the next person could be shyer and rarely smile but still be generally content. Some people might even smile when they are actually expressing a different emotion altogether, for instance some people will nervously smile when they are embarrassed. We learn to read individual people by making observations not just from their face, but their general body language and behavior, observing their personality and habits over time, and so forth.
As an example of the differences in cats, my current cat puffs his tail quite easily if something makes him jump, yet at the same time, he is definitely a lot less jumpy and anxious than my previous cat who rarely if ever puffed her tail.
As to what your cat is feeling, obviously I cannot observe your cat myself, so I cannot say with much certainty, but my current guess is that what you are observing is your cat's neutral posture. A cat that's actively trying to express anxiety or worry is unlikely to perk their tail up to the happy, confident posture just because they were petted. A worried or anxious cat will usually also behave in a way that makes their anxiety obvious. They'll warily slink about, act jumpy, and often try to hide.
But also the fact that your cat doesn't have its tail up more often in the happy, confident position says something about your cat's general personality. For instance, it may be that your cat is less expressive than some, or it may be that your cat is generally less outgoing and confident than some.