I'm pretty sure they intentionally delay "normal" parcels, just so they can charge a premium for the "express" ones.
Or perhaps they simply prioritize filling express orders first because the customer paid more for shipping and thus indicated a need to have it sooner. I know it nets out to the same observed effect but sounds like a reasonable way to do business. When you pay for a lesser class of shipping you are telling them you don't need it fast. In the face of an overwhelmed fulfillment system, prioritizing by shipping rate makes complete sense.
No sane company would intentionally delay a package. It gives them no benefit if the warehouse is not busy. I can not imagine a company doing that. As a small business selling and shipping products I have learned that you don't make much, if any, money on shipping. I'd be willing to bet the Mouser and Digikey MBAs have metrics that measure their order fulfillment system. Pretty sure the goal is to minimize the time between order and handing package to the shipper, regardless of shipping class.
I was talking about couriers and mail services - for those companies, there is an obvious commercial interest in making sure there is product differentiation between $100 Express and $20 Normal service, no matter how busy or not busy they are. All you have to do is make sure you make more money on that, than you lose in warehousing costs.
You would't even have to warehouse that much either. All you have to do is make the slower service "unreliable"... i.e. you would deliver faster if your warehouse levels begin to rise too fast, and slow down again when things are less busy...
You can see I am in the wrong career, right?