Questions · Evaluate
Do I need variant markup on the page if it is in the feed?
Yes, because agents do not all take the same path. An agent that arrived at a product page from a search result may never fetch your feed, and it should still be able to answer a question about size availability from the page it is on.
Why it happens
Shopping instructions are usually specific. A shopper asks for a size, a colour, a capacity. A product page that presents one price and one availability for a grid of twenty combinations forces the agent to guess which of those facts applies to the combination in question.
Guessing here is unusually costly, because a wrong answer about size availability produces a failed purchase rather than a mild disappointment. Agents avoid it by preferring stores that state the grid.
This comes up at the Evaluate stage. The full treatment, including how to reproduce it and what to change, is on Variants are not described in structured data.
Related questions
Run a free scan and find out which of these your store actually hits.