The stainless steel used in these tools is a very specific type of metal that contains specific additives to make it stronger and more resistant to rust. Specifically, 420 High Carbon steel is a version of 420 steel that has a higher percentage of carbon added to it, which in turn provides more strength to the metal by distorting the metallic lattice just a bit, making it harder to break.
The 420 steel that the carbon is added to already had at least 12% chromium added to it. The chromium provides a measure of protection against rust and deterioration, making the steel stainless steel.
So combining 420 stainless steel with the right amount of carbon allows the Leatherman company to create a tool that is not only resistant to corrosion, but also very durable. With proper care and maintenance, the knives of your Leatherman tool should remain sharp, and the tools should remain usable for the foreseeable future.