One number describes what the system could throw at a breaker. The other is what the breaker has actually been proven to survive. Confusing the two is how equipment gets asked to interrupt more than it can handle.
A breaker or fuse doesn't just have to carry normal load current — at some point in its life, it may have to interrupt a bolted fault. Whether it can do that safely depends on comparing two entirely different kinds of numbers: one that describes the electrical system at a specific point, and one that's stamped on the equipment itself. Mixing them up, or assuming one implies the other, is one of the more consequential mistakes in overcurrent protection.