Other than pre-production units, these

Watches have almost certainly not yet been manufactured.
I'm not sure why you seem to think it so inconceivable that Apple might be aware of improvements in battery density/technology that were not ready in time for the first units, but that are just about to hit the market, and which newer batteries Apple plans to incorporate in the final product, but simply have not yet been able to test extensively for stamina, and by extension product battery life specs.
Unless you manufacture all your own components, releasing new products, and sustaining an adequate pipeline for that matter, is a delicate balance between market deadlines and availability of screens, chips, batteries, and other various components, etc.
Edit: Just noticed a
new post, reinforcing possibly coming battery-life improvements before the

Watch's debut. (either by software
or hardware).