Why does hoverdata always have two curvenumbers?

The title says it all:
I have print hoverdata of a graph with several traces by

@app.callback(Output('image-overlay', 'src'),
              [Input('migration_data', 'hoverData')],
def update_image_overlay(hoverData, image_dict):

and get a result like

{'points': [{'curveNumber': 901, 'pointNumber': 32, 'pointIndex': 32, 'x': -108.5530990008, 'y': 54.943517268, 'hovertext': 'B4_6770_2'}, {'curveNumber': 691, 'pointNumber': 43, 'pointIndex': 43, 'x': -109.0138656987, 'y': 93.2052994555, 'hovertext': 'B2_3389_2'}]}

Why are there two curves here, and which is the one that I want associated data to be displayed to?