The fallacy of adding probabilities

I’m an avid reader of Maximum PC and have had a subscription since they were known as boot back in the late 90’s. (I’ve remained a subscriber despite the fact that they put most of their content online, a good move nonetheless, but I still like the printed version). They’re fairly accurate in most of their reviews and I respect them for both this and their useful guides and other features.

However, when reading their review of the FragBox II, I came across an interesting quote about failure rates: (Emphasis mine)

So why would Falcon configure RAM in single-channel mode? Falcon gave us three reasons for this decision: There’s only a minimal performance advantage to running dual-channel mode with this box; RAM is the second-most-likely component to fail (the GPU is first), so using just one DIMM cuts the chance of failure in half…

The emphasized part of the statement is wrong in the general case. But why?