I think what may actually be happening is Brad & JoAnn, the founders of MBC have spoiled us.
They operated with "customer service" as core of their business model and worked to have over 4 million bullets on hand so any order could be shipped out same day for USPS flat rate shipping pick up time. But 15-20 years ago, most other bullet casters/manufacturers, especially small mom and pop operations would cast bullets on rotation and customers would receive their bullets after several weeks. What Brad and JoAnn "chose" to do by having millions of bullets on hand is not what most other casters did as their "business model" operations.
Now Brad and JoAnn have moved onto bullet casters heaven in the sky and apparently a family member has taken over the operation of MBC. During management transition, business decision could have been made to use up millions of bullets on hand to raise capital and switch to keeping smaller quantities of more popular calibers and weights for immediate shipping (While they continue to supply retail vendors like Graf & Sons).
So if you order directly from MBC and there are no boxes in calibers/weights ready to ship out, bullets will need to be cast to fill that particular order in entirety (Say you ordered 4 popular calibers/weights but 1 less popular caliber/weight, then the order won't be fulfilled until less popular bullets are cast).
When RMR was being incubated on THR as Jake bought large bulk lots of bullets to resell in smaller boxes while attending college, I told Jake about MBC business model of utmost customer service and fast shipping (Along with 5% THR discount MBC started). After some discussion and thought, Jake agreed RMR would follow in the footsteps of MBC and ship out bullet order the same day, provide utmost customer service while offering 5% THR discount (PM member for code).
After a decade, business model Brad & JoAnn started is working out well for Jake too. And yes, we are spoiled by RMR's fast shipping as well and many of us have taken it for granted and only become aware of it when there's slow shipping. It's like shipping by UPS vs FedEx ... One is fast/predictable and the other is ... well ... It will get there when it gets there.