Of course they say it isn't guaranteed. Otherwise someone would try to sue if Pre-Check was closed for some random reason or the passenger himself didn't document his KTN properly when making the reservation, etc.
My point is that it isn't a random experience. When I book using our KTNs, I know we'll get Pre-Check, and over dozens of flights to many locations, I always have. Calling it "random" implies that you have a good chance of it not working. As long as you documented it properly, that isn't the case.