Well count your blessings then.
Here's one possible scenario to explain this: Leafminer's lifecycle includes dropping to the ground after feeding cycle inside the leaves is complete and pupating. So this area may be heavily infested, and if they are opportunistic, they will emerge from the soil as adults, (mate), and lay eggs on the nearest suitable plants.
Another is that there may be beneficial predators in other areas of your garden that curtail their activity. They could be predatorial nematodes in the soil that wouldn't range very far from the immediate area. ...or perhaps you used something in this bed that killed the beneficial nematodes (or other beneficial soil organisms).