I believe it's caused by adaptive brightness:
01-25-2017 07:40 AM RE: Dell XPS 13 (9360) FHD, Adaptive Brightness
Issue ?
There is no "issue".
The Dell XPS 13 9360 ships with a design feature called
"content-adaptive brightness control" (CABC). This feature will change
the brightness of the screen depending on the content that is rendered
on the screen.
A number of customers do not like this design feature (me included, by
the way). As a result, Dell have made available a tool which allows
turning off / turning on of CABC ( www.dell.com/.../DriversDetails ).
This tool is _only_ available for systems with the QHD display, as,
apparently, the tool reconfigures the screen panel firmware.
For the record: personally, I am very happy with CABC turned off on my
QHD display. I am also very happy that there is no dynamic brightness
control (reacting to environment brightness) is implemented either.