Think of it as trying to get a good approximation to 'true time' with ordinary watches. They, like computer oscillators are always going to be fast or slow to varying degrees.
Let's say you start off the watch in sync with the news pips.
Now wait a few days.
- If you only have one watch, you know what the time is, but you do not know if it is correct.
- If you have two watches you don't know what the time is period.
- If you have 3, You can have a pretty good idea of the correct time until one breaks, then you are in situation 2.
- If you have four watches, you can a sure of having a good idea of the time even if one breaks.