Salmon sell by date seems far off

Quick question...hoping you good folks here might be able to shed some light. I bought a large fresh salmon filet on 3/23 and it had a sell by date of 4/1. That seems like an awfully long time for dead fish to sit in the fridge. I cooked it yesterday (3/26) and man was it fishy smelling. If you add a couple of days past the sell by date for cooking, that slab of meat could be well over 10 days old. How does this make sense?