All of the brewing books mention that yeast cultures adapt to the brewing conditions of the brewery in which it is being used. Big breweries maintain stocks that have their unique characteristics. Cultures used in home brewing apparently does the same according to these same texts.
How does White Labs or Wyeast maintain the different strains? Do they continuously obtain strains from the great breweries of the world? Surely they don't spend their time duplicating Pilsner Urquell or Sammy Smith's Porter. After a while you'd think they'd have nothing but agar ale and slant stout.