And if it's a software problem, then again the redundancy isn't helping you.

Basically you haven't succeeded in getting 5 independent tries in the problem -- the failures are highly correlated.

It's the same reasoning that led to the financial crisis -- you have a CDO or an MBS that has a bunch of different obligations so you think you've diversified away risk, but in actuality there is one big factor that causes everything to fail at once.