It does not look like you are using the same model HEF file. Please note every model can be optimized in different ways and therefore you can have multiple HEF files for the same model that will run at different FPS.
In your second run you should see the same FPS for each run with the different batch-sizes. This leads me to believe your model was compiled to multiple contexts. Please run the following command to find out:
hailortcli parse-hef resnet_v1_50.hef
Where did you get the HEF files from? If you take the HEF from the Hailo Model Zoo the FPS should be close to the basic_model.hef.