In current public versions of stock Android, the volume rockers change ringer volume by default unless audio is currently playing. (android.gadgethacks.com)
Optimization - Vibration no longer affects the features which reduce ringer volume when the phone is lifted or flipped to silence (10 - 10) (androidcentral.com)
Optimization — Vibration no longer affects the features which reduce ringer volume when the phone is lifted or flipped to silence (10 - 10) (xda-developers.com)