I just saw that your post got missed. What has happened since?
Yes, it's sort of like the difference between water resistant and water proof. The plants are disease resistant, less vulnerable, but they are not disease proof.
But I can't tell you that's what you have. I'm always amazed growing stuff from seed, start with identical seeds, that are essentially clones of each other, grow them in seemingly identical conditions, and get non-identical results. Some sprout faster, some grow faster, they don't even always look identical. Frequently if there is a "runt of the litter," things get worse and worse for it... its bigger sibs crowd it out, so then they are getting more light and it is getting shads, etc.
Sometimes there are accidents along the way, one gets more roots broken in the transplanting process, or whatever.
So I don't know if your wilty plant has some differences from the others in the plants or in the micro-level of conditions the plant is in.