Lauri,
The difference is drainage in both cases. If you have fast draining soil in a pot, the pot is the appropriate size for the root ball, and the pot can drain excess water out the bottom, you will be unlikely to see rot from "overwatering". The risk of overwatering in those conditions may be from leaching all the nutrients from the soil.

A couple of years ago when I was in SoCal, we had heavy winter rains. There was considerable worry about plumeria in the ground, especially with the heavy native soil. No one that I knew lost a plant to rot. There was apparently sufficient drainage in the prepared planting soil and/or the native soil to prevent cold, soggy soil at the root zone. Also, the roots extend much farther in the ground than they can in a pot, further keeping the entire root ball from being waterlogged.

Another reason can be how the plumeria are planted. As you know Bill and others use a slightly raised bed, which gives more drainage and reduces the chance for water to accumulate at the roots.

So it all depends on the soil, either in a pot or planting hole whether water becomes a problem.
Mike

Also, ground soil is bottomless!

Tony