Let's look at one diagram first:
And look these,
As we can see, there are many different styles of reed switch.
We can simply tell it by 3 styles. They are Normally Open, Normally close, Open and Close.
The inside structure of the reed switch is the key answer.
CiAn Electronics suggest that, for high quality Magnetic Switch, we have to use the 3 top brands of the world, to ensure higher stability. And we do the same as what we said. Long years of production proves this suggestion. Never try to use cheap reed switch, or else people have to pay more after waking up.