PTC fuses are very difficult to design in. Overall, I find they are usable only for gross overloads- many times their rated holding current.
High ambient temperature aggravates their trip point dropping lower, but also it's highest just off the reel and lowers after the first 1-3 operations:
"Upon cooling, the polymer reforms to its semi-crystallized state and the conductive pathways are reestablished. However, when the polymer recrystallizes it does not return immediately to the same base resistance. It does not compact as tightly as when it was pre-tripped, and therefore
the post trip resistance in most cases is typically 50% higher then the initial resistance. Note the post trip resistance increase is not a cumulative effect; additional tripping will not cause increases in resistance in excess of the first trip event. Further resistance recovery is possible if the device is conditioned by actively current cycling the device or passively heating the device below 85°C."
Bourns Technical Paper - Soldering Polymer PTC Resettable FusesSo Bourns is saying its resistance increases, which is basically the amount of heat for a given current, so they'll trip at a lower threshold than the datasheets. This is a trap and always best to get samples and evaluate the parts in the lab.