With autumn high production, you're producing food that has to sit around for, essentially, three to six weeks before it is consumed. (Some food produced in autumn lasts through late spring to support the cities before higher production kicks in and starts supporting them.) This means the food is subject to anywhere from 20-40 days of rot and shrinkage. On non-season islands, that large stockpile and extended rot cycle doesn't happen. You can manage food on a weekly basis, and generally don't need to stockpile for 6 weeks of consumption. Therefore you don't get hit with all those extra rot cycles.
Poll