That's because we are all made different! Dosages will always be different for everyone.
Don't focus on the amount of the drug that you are getting right now, just be glad that the current dosage that you are on IS WORKING for you!
There most likely will come a time in your life that your dosage will change and that is why it is very important to keep all of your pdoc appts and let him/her know of any issues/difficulties that may arise with your current dosage.
Honestly...be glad that you found something for now that works well for you. It can take many of us a long time and some years to find that correct balance!