Weak links work harden with use, which results in a well used one having a higher breaking strain than a fresh one. Its not unusual to have a weak link last months, only to then have two or three break replacements in quick succession, until one survives long enough to get enough launches to also be work hardened.
The Air Cadets had a policy at one time of giving pilots a new weak link for a first solo flight, which was just about the worst thing that one could do, as it would fail at a lower tension than the cadet and winch driver was used to on the previous launches.