Questions · Buy
Why is a store that sells only the default size not a stall?
Because the purchase completes. The agent reaches payment with a product in the cart, so the buy stage passes and benchmarks do not count it as a stall. It is reported so you can fix the options, because a shopper who wanted a different size is not served.
Why it happens
Themes commonly preselect the first available variant so that add to cart works the moment the page loads. That keeps a purchase possible even when the option controls are built in a way an agent cannot operate, such as options that only respond to a hover or controls built from elements that are not form controls.
The agent can see the other options and cannot select them, so it can only ever buy the default. Whether that is the product the shopper wanted depends on the default.
This comes up at the Buy stage. The full treatment, including how to reproduce it and what to change, is on An agent can buy only the option already selected.
Related questions
Run a free scan and find out which of these your store actually hits.