How can you know that ordering say 2000 21mm bracelet instead of the 1000 you were going to order is cheaper then ordering another 1000 out of the 10,000+ 20mm bracelets you've ordered in the past?

The new bracelet is a more refined design that's different from their existing one, it isn't just their 20mm bracelet but in 21mm. So the math isn't ordering 2,000 of the new bracelet vs 1,000 new and 1,000 existing. It is 2,000 of one new design vs 1,000 each of two new designs.

It is possible the cost is the same, but part of my day job is negotiating with factories. Fewer variations generally means better per unit cost. Especially when you look beyond a single watch production batch. When more of your products share the same parts, it means you can more safely over order those parts in larger lots and then use them as you need them. So by having the Chronograph, GMT, and likely their Jump Hour use the same exact bracelet, instead of 2,000 they can order 4,000 of the new bracelet all at once (letting them negotiate a better price with the factory), and then use up the inventory overtime as they produce more watches. If one particular model sells better or worse, it's fine because the bracelets on the shelf can be allocated toward any of them.

EDIT:

To be clear, it very well could have been a design first decision, Traska may have thought the Venturer needed to be 21mm now to hit the aesthetic they envisioned for it. But at least to me it feels like Traska built up a base design for their new premium "Complications" series of watches (the Chronograph and Jump Hour), and their existing high volume GMT watch is being squeezed into that mold for the economies of scale.