I did some digging earlier, but don't have the link handy still. But found a website that had broken down the percentages for getting X hits on Y dice. 7 hits on 13 dice had jsomething like a 5.6% chance of happening. So it's basically like rolling a natural 20 in D&D.
Anyway, I reiterate again that the adventures are written as they are written, generally designed to be tough but not horribly so. However, it is up to each Gamemaster to judge both the difficulty of the adventure as well as the capability of the players, and to modify the adventure as needed to provide a challenge to each group of PCs.
Frankly, in an ideal world, I could not put in stats at all into an adventure, just put in a list of NPCs you'll encounter (i.e. 3 Lone Star officers), and the adventure would run just the same as if I had provided balanced stats, completely "average" stats (3's across the board), or built them to be Street legend killers (5's and 6's with skills in the double digits). because the GM would take each encounter and adjust it to fit the group he's running for. Alas, it doesn't work that way, but that's the ideal outcome we'd get

The stats we do give just provide a baseline to ballpark things from.