This is most likely a documentation-purpose difference rather than a hidden technical contradiction.

Published tech specs are often simplified marketing/sales documents. They tend to present the widest usable headline range in a clean way, because that helps comparisons and checklists. Calling out part of the top end as limited, special-purpose, or less optimal usually doesn’t help that goal.

The camera’s own menus and warnings serve a different purpose: helping you choose settings in real shooting. There, it makes sense to flag some ISO values as “extended” so users know they may involve compromises and should be chosen deliberately.

So the most likely explanation is that the spec sheet reports the broad selectable range, while the camera interface gives more nuanced guidance about which settings are considered beyond the core range. In short: the specs emphasize marketable range; the camera emphasizes practical use.