Shoot Low and Act Surpirsed

Note the following NASA article: http://science.nasa.gov/headlines/y2009/31dec_uncertainfuture.htm?list912666. Specifically, the sentence that notes "The rovers began missions intended to last for just three months but which have instead gone on for six Earth years, or 3.2 Mars years." Why do they do that? It seems every mission that has ever been sent into space, from the Hubble to Voyager, has been intended to last some ridiculously short amount of time (not to mention ISS or Skylab or Mir). And, it's like a suprise that they make these things last for decades longer. Wow, we thought we sent up something crappy into space, but wow, it is really still working. Must have been designed by engineers or something. Not marketers. Hubble, launched in 1990, was supposed to have a life of something like 5 years, but 20 years later, it's still going. Yes, there have been multiple service trips, but it appears that, after the initial mirror repair, most of the service calls were just to update equipment and swap out some insulation. They make it sound originally like after some seemingly random time period (how they arrived at that specific time frame is never explained), there will be some catastrophic power failure and the piece of tech will just die. And it never happens. So, what I'm wondering is whether the ridiculously short time frames are just to cover their behinds (in case something goes wrong, they can always say, well, it was supposed to happen 20 years ago), or whether these are valid surprises as far as machines inexplicably working years past their alleged expiration dates in the dead of space.