When is a Dynamic Port "dynamic"?

I was having a discussion today about Dynamic Ports with one of my co-workers and could use some help clarifying how they work. **First question:** If the IPALL TCP Dynmaic Ports setting is a specific number (say 1971) does that signify that you have a static port of 1971 or a dynamic port that is currently 1971 and may change at some point in the future. **Second question:** This is the one I'm most curious about. We have an instance that has had the same port (the value in the IPALL TCP Dynmaic Ports setting) for several years through dozens of instance restarts. What actually causes the dynamic port to actually change after an instance restart?