I think I see what you're asking there Kev.
It's
generally accepted that
most but
not all rivers have Salmon entering the system every month of the year, but typically these runs are grouped into 'spring''summer' and back-end runs. Certainly, there is some continuity in terms of numbers and times fish run which is why we have those terms at all.
It's not exclusively true, but springers do tend to spawn in different places than the other runs of fish.
They do indeed stay in the system until a common spawing time, but again it's generally accepted that they do spawn a wee bit earlier than their later running counterparts, but not much.
If they survive spawning and make it back to sea, then they do indeed return a year later - so they miss the next years 'run' as you suggest.
Cool isn't it
Hamish