The laptop ships with a feature called Content Adaptive Brightness Control (CABC), which conserves power by dynamically changing the display's brightness setting depending on what's being shown on the screen.
While the feature can be annoying for most of us, it can render the laptop useless to multimedia professionals who aren't able to calibrate the display due to the constant fluctuations in brightness.
Because CABC is controlled in the Bios, there used to be no way to switch it off - until now. Dell has issued a patch that disables the feature, although you'll have to contact the company's customer support to get hold of it.
If you're rocking Dell's new Skylake-equipped XPS 15, don't worry – the XPS 13's big brother doesn't use CABC, so there's no action required there.