I'm not quite sure what you are asking, but it's common to get different effects from the same doses of the same pills at different times in your life. I find that to be especially true with the Benzos, and again, especially true when you're using them for true medical purposes and not recreationally.
With you, are you taking any other drugs that may be interacting with the
Xanax? Maybe you were taking it in the past and you quit now? That can even include caffeine, especially in tremendous amounts like 15 cups of coffee a day. If you were drinking that and no longer do, that could account for why the Xanax is so much more effective. I've also found certain generic brands of drugs to either be much weaker, or occasionally stronger. Then there is you. Maybe this is a very calm time in your life, and that has something to do with it. I can't be sure from your name, but if you're a woman your monthly hormone levels can also effect this I think. I know it effects alcohol.
So I can't tell you for certain why, but I can tell you I've had similar experiences with
Valium. I've used Valium on and off for over 20 years. There have been times when 10 mg was all I needed to relax me and there were other times when even 30 mg a day wouldn't calm me down.