Have you ever tried to set a digital timer for 100 seconds? Yeah, probably not; I’m weird.
Anyway, you can’t. It defaults to one minute. Thus taking you from 100 to 59 seconds.
It does work to try 99 seconds, but 101 will get you 61 seconds. So, if you want to nuke your food for 100 seconds, you will have to settle for 99.
Math has never been my greatest subject.