Deep Blue rating has very wide bars due to many facts:
* very low game count = 6
* Kasparov used Pentium 166MMX with some strong engine of that time (don't remember which one)
* specially for this match, Kasparov didn't play naturally at his best. He had prejudice on computer play style, based on his previous experience with other computers and decided to play what he believed "anti-computer" moves.
Kasparov attributed 44... Rd1 to "superior intelligence", though it is 17th worst out of 25 legal moves and forced M17 for machine
* during most of human-maching matches - special adjudication rule was used: "resign as early as possible / offer draw as early as possible". Humans are not prone to blunders and to fatigue, so this rule favours humans a lot.
here are few multiPVs from Deep Blue matches:
(1997, Game 2. Kasparov resigned in draw position)