difference in using multiple -d for curl vs just concatenating data with &

I am not sure which one is the correct way to use curl between the two methods below.

  1. curl --data "api_username='user'&api_key='key'" http://localhost:8083/

vs when curl is used as

 2. curl localhost:8083/ POST -d api_username='user' -d api_key='key'

My problem is with regards to how to specify the data being sent to the server, 2 seems to be working however I am seeing 1 is sending some extra data.