This is something that Views does when a particular field is used in more than one Entity type. In your case, the "Abbreviation" field is used both by node entities (the "Content" views group) and by taxonomy term entities.
It represents the field in both the "Content" and the "Taxonomy Term" group, so that if you were filtering the list by group, you wouldn't be missing it in the list. But it also represents itself canonically in the "Field" group, to correctly inform the user that the field is available in multiple entity type contexts. This would be important knowledge if you had both entity types available in your View because of a relationship.