What you caught was not a jack, it was a Jill. Although rare, they do exist. I caught one some 8-9 years ago and was asking myself the same question. At the time, this was the best response I could find:
C&P from another forum:
A 'jack' salmon is a sexually mature male salmon that returns to spawn one year earlier than his 'normal' year class. A 'jill' is the female equivalent and although not as common as jacks, they do exist. Jacks and Jills are obviously a smaller size than those salmon living out their entire life cycles since they spend one less year feeding in the ocean. The size difference is most notable with coho as returning jacks/jills more resemble small trout than salmon. As we know, cohos put on the bulk of their size in the last 4-5 months of their normal life cycle. Cohos or bluebacks are typically in the 2-4 lb range in May but will reach upwards of 20 pounds by Sep/Oct.
Jacks and Jills occur in all salmon species with the exception of Pinks. Its not entirely known why they don't exist in Pink stocks other than the fact that they already have a very short life cycle (2 years). Jacks/Jills are very common in Sockeye. One of the reasons why we don't really notice them is even an early returning 3 year old fish can easily be 3-5 lbs. The next time you catch a sockeye that seems a bit on the small side, it could very well be a jack/jill.
Early returning Chum salmon (3 year olds) are normally not called Jacks/Jills as 3 year olds make up a fairly significant proportion of normal 4 and 5 year old returns. They could be called jacks/jills though if our definition of such is 'a fish returning one year earlier than the majority of its cohorts'.
The purpose of a jack/jill salmon is, like its big brothers and sisters, to propogate. Although Jills will find ready 'dance partners' with mature males, Jacks aren't so lucky. Jacks are usually chased off by larger males and either prevented from spawning totally or, they find a way to scoot in on the action and deposit their cargo (adding to the genetic mix). In fish stocks that might be heavily exploited commercially, Jack salmon might be the difference between a successful spawning season and an unsuccessful one (if too many mature males are harvested).
Finally, Jacks/Jills do not 'begat' more Jacks and Jills in the future, just because they spawn. Early sexual maturity in salmon is more an act of randomness than anything else. So the next time you catch a Jack, release it with respect and let the lil guy do his thing.