Dropout occurs, period. Unless one is on the net 60/60/24/7 some fish will drop. Way back when (80s) there were locations where the pinnipeds net fished. They drove fish into the nets and took them out. Even with the netter trying to chase them. I can't imagine in 30 years that seals got dumber.

A gill net has an optimum size of fish it will retain. That is why there are (or should be) sockeye, coho, chum, and chinook nets. A fish that is outside the target range may tangle, snag a tooth, or whatever. Then, die and come un-netted.