How could I be addicted to fioricet but have been taking Xanax (alprazolam) for panic and have never abused it/felt the urge?

Both are addictive. If you are getting Xanax (alprazolam) on a regular basis you might not have known that you were addicted. Try stopping for a number of days, a few weeks, see if you have the urge for that as well. Barbiturates (fioricet) and benzodiazepines (xanax) are both "downers" and both have tachyphylaxis (habituation) qualities.
Different type. Fioricet has caffeine which is addictive vs Xanax (alprazolam) which is benzo. Xanax (alprazolam) addiction is subtle and withdrawal effects are prolonged.