the increase to 37% from 34% cannot explain all of the margin compression, this is a 3% re-allocation of the "pie" unless of course, this new added business is a loss-leader which is particularly troublesome.
I don;t think you can gleen anything telling in the sales mix. What clearly stand out is the sg&a costs. I beleive if you read the 10q you'll find they explain the resean behind the increase. They also sight increased fuel costs as having a 22 basis point effect.
All in all, it is given that as their sales growth moves into the major's, they will see some mrgn erosion. However, as is the case for almost all that sell to the major, it should be made up with volume
Your assumption that sales to majors will be at lower margins for UNFI are incorrect. They pay list prices and are given ongoing deals from manufacturers that are specific to each retailer. The sales to majors are made through many stores in a chain with small amounts of product to each store. The sales to Whole Foods are multiple pallets and even truckloads to each store. The sales to supermarkets are partial pallets to each store... It's still good business, it's just different...