ADS1115 differential measurements - resolution limited to 12 bits instead of 16 bits
Edit:
This seems like something you're doing wrong with the library or the library has an issue.
Check that  you have the latest version ... there is a .1115 header file available. If the software thinks you have a 1015 you'll get the results you are getting.

Did you set this correctly?:
  /* Be sure to update this value based on the IC and the gain settings! */

  float   multiplier = 3.0F;    /* ADS1015 @ +/- 6.144V gain (12-bit results) */
  //float multiplier = 0.1875F; /* ADS1115  @ +/- 6.144V gain (16-bit results) */