Different processors made in different plants at different times with different materials perform differently. Some do better, others do worse. Additionally, later processors tend to do better. The stepping is a code printed/etched on the core of the processor to identify the situation in which it was produced so that the company can figure out what went wrong if all of a sudden a huge number of processors are RMA'd.
On an athlon XP, the stepping will be something like AGKGA or AGOGA or AGNHA. If you take off your heatsink and clean the core really well, you'll see where it is. (Just be sure you have some more thermal compound before doing this.)