You haven't described your monitor. Is it an LCD? Does it have its own setup menu in the hardware, to auto-adjust what is presented based on ambient lighting or other characteristics of what is being displayed, etc.? Can that be suppressed?
Does it have "auto-adjust" software (probably through a separate USB connection to the monitor used by the adjusting software, in addition to the DVI/HDMI connection)? This too can either do something or not, which can affect how things look on the monitor.
For example, I have an Eizo monitor which DOES have controlling software that runs on the PC named "Screenmanager Pro". This can control how individual program windows look, including the ability to modify brightness, contrast, color, gamma, etc., when that program's window has the focus. Various presets are available, or you can create your own "custom" settings.
This Screenmanager Pro software operates in addition to the monitor hardware itself working to auto-adjust what is shown.
Anyway, it is very common for windows controlled by this software to look different from the full-screen presentation of the same image content.
So... might your own monitor be the "culprit" in why image rendering in a window looks different than image rendering in full-screen?