My guess, your unit is in a city center. Base healing rate is 10 in neutral territory, 5 in enemy territory, 15 in friendly, and 20 in a city. The medic's modifiers only add to that base number, which makes them useful because without medics it is basically impossible to heal units in enemy lands. You unit, if it's in a city, is healing 20 base + 3 from your modifier = 23.