The body gets ill when it needs to basically get cleaned up.
Many people only experience partial symptoms to full exposure of a virus or bacteria. For example, when my daughter had full exposure to chickenpox, she got the fever right on schedule (so many days after exposure) but only developed one or 2 spots that barely looked like chickenpox. I would have never thought that she had chickenpox if I weren't looking for symptoms and knew what day to expect symptoms.
So you probably got full immunity to the mumps, but didn't develop the typical swelling in the jaw area. Maybe you simply felt a bit under the weather.
It is also interesting to note that almost everybody exposed to polio simply gets cold symptoms - a few sniffles.