I think the 70% is because there are so many more things to go wrong with software, they can be little or small. But you'll run about... I don't know, a hundred things in one session on your PC that are software. About seven or eight pieces of hardware, maximum of ten or so. The hardware doesn't change, it just runs until it breaks. You could install a new piece of software on your computer and the whole system can be different, a new range of bugs could popup. You get variables that you change... so much can go wrong with Software, at design level even. Piece of hardware is physical, you won't get a bug in a piece of hardware very often because it would have blown it up when they tested it, or when you switched it on.
Down to how many things could go wrong I suppose.