I should have left off the words, "to the surface".
During the seasoning process ... the oil fills in the voids in the surface of the cast iron and "fuses" to it. But, it also goes thru a polymerization process where the oil actually changes and forms a hard solid surface. If you've varnished anything it's kind of the same thing ... as it dries it gets tacky - then begins to form a cohesive layer which is still soft, and then turns into a hard surface. Kind of the same thing that happens when you break in a new car's engine.
"It ain't what you don't know that gets you in trouble. It's what you know for sure that just ain't so." - Mark Twain