With that being said, it seems especially surprising that Mark Buehrle would throw one. He is a real good pitcher, but his greatest strength is consistentcy. He always gives the White Sox quality starts, but there is not much of a range to his performances. He rarely has a brilliant performance, but also rarely has a bad one. Every start is about as good as the last.
It's not hard to theoretically figure out how likely it is for a pitcher to throw a perfect game though. The question boils down to how likely a pitcher is to get 27 outs in a row. On-base percentage is essentially the chance a player does not make an out, so taking 1 and subracting Buehrle's opponents' on-base percentage from it gives the chance that Buehrle gets any one hitter out. Raise that to the 27th power, and you have the theoretical chance that Buehrle gets a perfect game in his career.
For his career, Buehrle has held opponents to a .313 on-base percentage. That means he has a .687 chance of getting an out against a typical batter he has faced in his career. Based on that number alone, he should pitch a perfect game 1 out of every 25,245 starts. Then again, this is only the chance he gets 27 outs in a row, so you could further argue that the odds are slimmer. For instance, he could retire the final batter of his previous start, then retire 26 in a row and get a hit in his next start, or get the final 2 outs in his previous start...you get the picture. The odds are slim.
However, Buehrle is holding opponents to a .301 on-base against this year. Still, based on his 2009 numbers alone, it was roughly a 15,187 to 1 chance. Considering he definitely won't get any more than 40 starts (including any possible playoff ones) this year, the odds are still razor thin. Moreover, that's assuming Buehrle was facing a team with a league-average on-base percentage. The Rays as a team have a .353 OBP, while the league average is .330. Accounting for that, and the fact that Buehrle retired the final batter in his previous start, you could argue the odds were as miniscule as 55,683 to 1 that Buerhle would throw a perfect game today.
No matter how you slice or dice it, a perfect game is special. The odds aren't so ridiculous that they should never happen. That's the beauty of playing so many games every year. However, no pitcher in major league history has ever had nearly enough starts to reasonably say they should throw a perfect game.
To prove how ridiculously good a pitcher would have to be to expect a perfect game, let's make a hypothetical one. Let's say our pitcher made 35 starts for 30 years in a row. This pitcher is a durable workhorse to say the least. That would be a total of 1050 starts in his career. This theoretical pitcher would have to hold opponents to .227 on-base percentage for his entire career to likely throw a perfect game. That's equivalent to approximately a 0.68 WHIP, again, for an entire 30-year starting career. Mark Buehrle may not seem like the type to throw one, but frankly no pitcher has ever been the type.
A perfect game takes a perfect storm. While being a better pitcher greatly increases the odds, and facing a bad offense does too, it's the difference between lightning striking once or twice. Theoretically, lightning striking once has a much better chance of happening - but in reality, it's hard to see a difference because both are so remote.