They base it on a few things, like your weight, family history for blood pressure and blood clots, in your case whether or not you are breastfeeding, history of migraines, history of side effects on different pills (if you have used them before, which you have).
I think doctor's will each have their own preferance too. They may take into account cost or insurance. So a doctor who mainly sees woman without health insurance may be more prone to recommend a cheaper pill. If you went to five different doctors without having a whole lot of health concerns or restrictions you would probally get four or five different prescriptions.
They base it on a few things, like your weight, family history for blood pressure and blood clots, in your case whether or not you are breastfeeding, history of migraines, history of side effects on different pills (if you have used them before, which you have).
Another thing a conscientious doctor might consider is how heavy your normal periods are.