Quite a few popular strains started out as unknown crosses, often accidentally. When there is a live plant whose lineage is not known, seeds of that exact cross cannot be created and the only way to get more of that same plant is to clone it. These are clone-only strains. G13 started out as clone-only because, as the story goes, it was bred by the government. Since it was discovered, growers have backcrossed children of G13 and similar plants to create seeds that are close to the original. None are exactly the same as original G, with subtle differences based on what plants each G13 was crossed with. There are even a few breeders (way more than there should be) that have no idea what original G13 was and sell seeds that they call G13, but that are absolutely nothing like original G. Because of all the crosses and misrepresented strains, we can't speak intelligently about any seeds labeled as G13.
Personally, the first time I smoked G13 about 15yrs ago turned out to be the most memorable pot I ever smoked in my life.