Yeah, isn't it like practicing? You're not very good at something so you practice over and over and over and hopefully when you're done you do it better... You know different than when you started.
Technically, even then doing the same can lead to different results, if nondeterministic events play a role and the different aspects of the software or system may contain bugs. For example mutlithreaded applications where the scheduler can passively influence the outcome of an operation. In one run it fails, in another it doesn't. A nightmare to debug.