mortgage payment decrease

Why does my mortgage payment decrease by 3% year after after?

The mortgage payments on my primary home and on my rental both decrease by 3% year after year, but my mortgage is fixed, so the total payment doesn't change.

Why is the system reducing my payment like this? It doesn't show a principal/interest breakdown, which could make sense if it had asked me for my rate and was showing the interest only portion, but the first year's payment is both principal and interest.