Later maps will generate more experience points, which in turn means your stamina go up as you level a bit faster, so at some point, you'll reach the point where returning to those old maps to get 3 medals out of them costs (relatively) "small change" in stamina, so you start doing them again to burn your last stamina before heading to work or going to sleep or something.
May sound odd, but when you have 12-3 on farm (35 stamina), you care more about the Secret Garden that might spawn at its end (50 stamina), and at times end up with odd leftovers, like 60 stamina just a few minutes before you have to log for several hours. And, since you probably already completed 12-3 once you have it on farm, and don't have the stamina to deal with a possible Secret Garden spawning after it, that 60 left-over stamina might as well be used for one or two attempts at a lower level zone that was only "cleared" but not "completed".