The steady-state fault current from a short-circuit study is never the worst moment of the fault. The worst moment is the first half-cycle — and how much worse depends on the X/R ratio.
Run a short-circuit study and you get a clean symmetrical RMS fault current number — the steady-state value the system settles into. It's tempting to treat that number as the worst case a breaker will ever see. It isn't. For the first several cycles after a fault occurs, the actual current can ride well above that symmetrical value, lopsided and asymmetrical, before settling down. How much higher, and for how long, comes down to a single ratio: X/R.