This MacBook Pro uses a subwoofer speaker in addition to the left and right speakers to enhance the midrange sound characteristics.
You may notice that when you move the Balance slider in the Sound preferences pane far to the left speaker that sound still comes from the right side of your MacBook Pro. This sound is generated by the subwoofer, but because of its placement, the audio from the subwoofer may seem to come from the right side speaker. Due to the placement of the subwoofer it is normal for the sound stage to seem shifted slightly towards the right speaker. If you prefer, you can adjust the balance control to shift the sound stage to the left or right side.
The subwoofer is located to the right of the spacebar. The locations of all the speakers are shown in the image below.
There are no settings in the Sound pane of System Preferences to adjust the subwoofer speaker independently of the left and right speakers. Increasing and decreasing the system volume adjusts the left, right, and subwoofer speakers at the same time.