I see additional properties in my Structured Output

I have defined a JSON Schema of an object having a “entityId”, “name” and “description” property. When I get a structured output back, I’m seeing these filled but sometimes I also see additional properties (such as “price”).

I’m a bit puzzled because I would expect the Structured Output would guarantee that returned objects are following the schema. Is that not the case? It does include the three properties I have in my schema so it does conform in that sense … but I would expect not to sometimes get another “price” property, especially considering we explicitly have to set the “additionalProperties” to false.

Thoughts?