Portable headphone amplifier - max output and power supply
It looks as if the unit has a bipolar DC-DC boost converter within.  They specifically mention plus minus 7V rails.

Regarding the output voltage maximum specs: my best guess is that they are running into current limitations of their power amp stages.  As far as I am concerned, this is a reasonable trade-off.  Those voltage levels into headphones or In Ear Monitor (IEM) buds will be painfully loud.