When I look in label_annotions of the Google Vision API, the "score" and "topicality" field values are always the same. This is also for example the case here. According to this reference topicality refers to "the relevancy of the ICA (Image Content Annotation) label to the image" whereas score has replaced "confidence". Though it's now not so clear to me what "score" actually means.
Are these supposed to be always the same? What does that mean?
As you well mentioned, these "score" and "topicality" fields always display the same value. Based on my understanding, this is an existing bug on the Vision API functionality that needs to be fixed in a future release. I recommend you to take a look on this ticket that has been created to review this issue, as well as the Release Notes documentation of Vision API to keep the track of the new functionalities added to the service.