A "30-day supply" on a pill bottle is a marketing decision, not a medical one. It's sized to get you to the next order before you've had time to decide the first one didn't do much. Distributors know this pattern well — it's the same reason some suppliers push a minimum order quantity that's larger than what a buyer actually needs.
The difference is what happens after. A distributor who oversells stock has to answer for it — it comes back, or it doesn't get reordered. A pill brand that oversells a result mostly just loses one customer among thousands and keeps running the same ad to the next one. There's no return path for a disappointing capsule the way there is for a bad batch of stock.
That's one reason plain food holds up better as a long-term habit than a rotating cast of supplement brands. A box of dates does the same thing every time you buy it — same taste, same texture, same one-word ingredient list. There's nothing to be disappointed by because there was never an inflated promise attached to it.
If you do want to try something new, buy the smallest amount available first, the way a careful distributor test-orders a new supplier before committing to volume. If a company won't sell you a trial size and only offers a 3-month "value bundle," that's worth noticing on its own.
Judge what you buy by what it actually is, not by the size of the bottle or the length of the subscription.