They say "minimum" to appeal to lowest common denominator. if those reqs are too high then people who don't know any better may not buy the game at all, costing them sales. The publisher would much rather sell the game to some one and have it run like crap at lowest settings than not sell that copy at all. I can't believe they only said 512mb of ram, tho,for XP. The OS alone barely runs on that! That's pretty evil!
Recommended reqs, tho, can be a problem, because they seem to grossly under estimate those.