The battery figure is derived from a standardised test, though:
Laptop Battery Test
This test, developed in the Laptop labs, replicates continuous Web surfing over Wi-Fi until the battery is completely drained. Starting with a full battery, a notebook runs a script that visits 60 popular websites in a loop, pausing for 30 seconds on each, then closing and reopening the notebook's native browser with the next page. The test is run with the screen at 100 nits (as measured by the XRite colorimeter), and the notebook's settings are tweaked to prevent it from entering standby mode or going into hibernation.
I was disappointed by the result, if only because the standard XPS 13 is, well, the standard for ultrabook longevity. The trend of trading off battery life for portability on Y-series models seems to be industry-wide, and I'm not a fan. Such devices may last a full day today, but how about two years down the road?