'Tolerances' is a common term in this forum and the same applies to tools. The less play in the size of the hex tool, and the less likely it will round out the inside of the screw, as it can't start to twist, which begins the process of stripping the screw.
Cheap hex tools are made with looser tolerances, so the budget makers err on the side of caution by setting their process to make all the hex tools slightly smaller than the correct spec. This way the tool that are too big for spec, coming out of the factory, still fit, and the ones that are at spec or smaller fit as well and no one returns the tools or complains. You can imagine if 50% of your output was slightly too big for the hex hole, the problems you would have, so the cheap hex tools are typically mostly undersized. People who use them end up thinking the scews are bad. Wiha makes high tolerance tools that are the right size, and are less likely to start the process of the inside getting rounded out of your screws