Gary the major reason for the higher efficiencies is becsue you can create a convoluted path for the flue gases with a coal unit. Flue temperatures are much lower on a coal unit, unless it just got done stoking I can place my hand on my flue pipe, you aren't going to do that with a wood stove. For example take this cutaway diagram for a Keystoker boiler, note how low the exit for the flue gases is. The bed where the coal is burning is above where the gases exit and pulled through that internal water jacket before exiting.
Let's take my Van Wert as another example, note the door for accessing the fire pot is slightly lower than the height af the flue pipe:
When you ope that access door you don't see any exit for the flue pipe do you.
If look at side view you can get a better idea why, you can't see it in this picture but the box extending on the back is just for the flue gases. Internally a plate extends below the fire pot that the gases have to go below before they can then go up and exit into the flue pipe.