Reliability of a service
Ignore the software reliability for now, and only focus on the machines.

In case 1 there is only one machine that can fail, and it fails 1% of the time.

In case 2 there are two machines that can fail. However, even if just one fails the service goes down. The chance that at least one machine fails is $1 - 0.99^2 = 1.99\%$. So instead of spreading the risk we've just made our service more liable to fail.

There are two components to making a system reliable despite hardware failure:


Spreading out risk of failure across multiple machines.
Making the system robust to failure of some machines.


In the exercise we do have #1 but are missing #2.