This is because the Taurus uses current limiting to avoid the output to become dangerous for the amp, headphones and the listener.
Regardless of that chart, the power hungry HE6 sounds better in balanced mode, so I suspect the current limiting is done in a way that primarily affects continuous RMS, or at least gives plenty of room for short term voltage swing (no idea how, though) I also find the Taurus better than the 8 Watt Mjolnir for HE-6, along with several speaker amps. So don't let that number scare you off ,)
Here's from an email from Auralic
"The power in chart are continous RMS.
For your first question, there are four output sections in each Taurus amp. In STD mode, each two of them are in parallel driving one channel. It will well suite low impedance as the output current is plenty but the output power driving high impedance is limited by maximum swing voltage. In BAL mode, two section are in bridged connection(BTL), it will offering 2x maximum swing voltage so the output power at high impedance is four times higher, but at low impedance, since each section is bear half load which equal to 16ohm, the output is limited by output current. I hope this will help.
Best regards"