Why are assigned PID numbers not sequential from 100?
Process numbers are assigned sequentially, starting with 1. The kernel is sort-of considered to be process #0, but it's not really a process, so this is a special case. It does not skip ahead to 100; that's something different. The gaps you see in the sequence are indeed due to processes that have finished. Most processes are sort-lived; they start, do some specific task, and then when the task is finished they exit.

As for that "100" figure, that has to do with what happens after 99,998 processes have started. That's the maximum PID, so at that point the system has "run out of numbers" and starts re-using them starting at 100. But this'll occur only after your computer has been running for quite a while, not during initial startup. It can take months or even years without a reboot before you run out of PIDs and restart from 100.