Oops, I accidentally deleted the parent post! To clarify the context to other readers, the point I made in it was that one extremely strong piece of evidence against Clippy's authenticity, regardless of any other considerations, would be that he displays the same level of intelligence as a smart human -- whereas the abilities of machines at particular tasks follow the rule quoted by Joshua above, so they're normally either far inferior or far superior to humans.

Now to address the above reply:

The second is very important: Do you mean about the middle of the pack or akin to the very best humans in the skill?

I think the point stands regardless of which level we use as the benchmark. If the task in question is something like playing chess, where different humans have very different abilities, then it can take a while for technology to progress from the level of novice/untalented humans to the level of top performers and beyond. However, it normally doesn't remain at any particular human level for a long time, and even then, there are clearly recognizable aspects of the skill in question where either the human or the machine is far superior. (For example, motor vehicles can easily outrace humans on flat ground, but they are still utterly inferior to humans on rugged terrain.)

Regarding your specific example of chess, your timeline of chess history is somewhat inaccurate, and the claim that "the best chess programs are still not always beating grandmasters" is false. The last match between a top-tier grandmaster, Michael Adams, and a top-tier specialized chess computer was played in 2005, and it ended with such humiliation for the human that no grandmaster has dared to challenge the truly best computers ever since. The following year, the world champion Kramnik failed to win a single game against a program running on an off-the-shelf four-processor box. Nowadays, the best any human could hope for is a draw achieved by utterly timid play, even against a $500 laptop, and grandmasters are starting to lose games against computers even in handicap matches where they enjoy initial advantages that are considered a sure win at master level and above.

Top-tier grandmasters could still reliably beat computers all until early-to-mid nineties, and the period of rough equivalence between top grandmasters and top computers lasted for only a few years -- from the development of Deep Blue in 1996 to sometime in the early 2000s. And even then, the differences between human and machine skills were very great in different aspects of the game -- computers were far better in tactical calculations, but inferior in long-term positional strategy, so there was never any true equivalence.

So, on the whole, I'd say that the history of computer chess confirms the stated rule.