I do the same thing. Moisture retention of the garden soil is simply better, Dave. At least that would be my first guess. The garden stays fairly moist (unless there isn't enough rain during the fall and winter months) and the soil in the pots regardless of pot size (mine are 24" in diameter), dry out much more readily. Also the soil in the pots will probably get a few degrees colder during the winter months simply because they are above ground. That may pay some role.
