Possible reasons for why the render result might be different from the preview.
HDR formats
A possible reason for why the render result is darker or lighter is when the render output is saved as an EXR file.
EXR files store linear color data without gamma correction. This means they preserve high dynamic range (HDR) information but can look darker when viewed in applications that expect gamma-adjusted images.
Other formats like PNG files and the Real-time View apply sRGB gamma correction (typically 2.2 gamma), making them look brighter and more like what you'd expect on a standard display.
In order to make the EXR more consistent, please apply a Apply a gamma correction (usually 2.2) when converting or displaying the image.
Custom Controls and Render Engine
When using Custom Controls, a different algorithm and lighting settings are being used which results in a different render result.
Specifying a different Render Engine than the one used in the Real-time View might also have slight differences since there currently is no complete parity between CPU and GPU materials.
Performance Mode
If the Performance Mode is enabled, it only affects the Real-time View, but not the rendering output.