Why does xarelto dosing go from 15mg 2xs a day for 21 days down to 20mg 1x a day?

Treatment of clot: This is the recommended dosing to treat an acute blood clot in the vein or lungs. The 15 mg twice daily is used for three weeks and then down to 20 mg daily. This dosing was shown to be effective to prevent the blood clot from enlarging.

Studied that way: Depending on the "indication" (reason for use) the dosing of Xerelto is different. If you're taking it for a blood clot, the dose is a little higher at first, when the clotting propagation (getting larger) risk is highest, and then can be reduced after things stabilize a little bit. This is a different dosing strategy than when it's taken for other reasons (like chronic atrial fibrillation)