Common issue
In stock in one place, out of stock in the other.
Stock is the value most likely to disagree, because it is published in several places at once and they do not always update together.
What usually causes it
Variant-level stock on a product-level page. A product with eight variants where half are sold out is neither in stock nor out of stock as a whole — it depends which variant is being described.
Markup that is not refreshed. Structured data can be cached, or written by an app that updates on a different schedule than the storefront itself.
A value nobody recognises. Google accepts a specific set of values. A page publishing something outside that set may be saying the right thing in an unusable way.
The page answering differently to different visitors. We measured this on real stores: the same product URL returned in stock to one request and out of stock to another, seconds apart, with only a request header different.
How to check it yourself
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Check the variant, not just the product
Find out which variant the feed row describes and whether that specific one is purchasable.
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Read what the page publishes
Look at the structured data, not only the visible button — a crawler reads the markup.
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Reload and compare
If the page gives you a different answer on a second visit, the problem is the page's consistency, not your inventory.