To be honest, I think one of the problems is that minimum specs are usually based on a clean and efficient systems with nothing running in the background, wheras the vast majority of computers arn't like this, and it's almost impossible to keep them that way. That said, you can't expect a program to run well on a minimum spec system if it's not been deragmented in months, has loads of things running in the background is cluttered up with spyware, and the case is so full of dust it's not being able to keep cool. Another problem is that programs are usually optimised for a certain operating system, so software designed for XP might not get as good performance on the same system running Vista and vice versa. Performance related updates for the OS and the program are also a big factor.