Think of the probability assignment of a piece of gear like a range. My experience with random number generators is that they return a fraction between 0 and 1. Lets say Thagomizers are assigned a success probability range of 0.1 to 0.2. The program generates a random number. If the random number is 0.15, it is within the Thagomizers probability range. Happy days. If the random number is 0.3, it's outside the range, no joy. So the random number generated is random. But the probability range for success could be assigned, and in fact weighted differently for different items. I have my suspicions that Gjallerhorn had a very narrow probability range.