Why does the STM32L4 ADC have a 12-bit SAR and 16-bit data register?
You are able to select an alignment within those 16 bits, if you want the ADC data to be in the most or least significant bits. 

If it's right aligned bits 15:12 will be 0, or the sign if it's signed data. It's documented in section 18.4.26 of the linked reference manual.