This usually happens because the JPEG contains an embedded ICC color profile that is incomplete or malformed. Color-managed apps may interpret that profile one way, while other viewers or browsers may ignore parts of it or fall back to a different assumption, making the image appear lighter or darker.

So the issue is likely not the pixels themselves, but inconsistent handling of the embedded profile.

A practical fix is to remove the problematic ICC profile, or re-save/export the image with a standard, valid color profile such as sRGB. That gives different software a clear, consistent color space to use and usually makes the image display more uniformly across viewers, browsers, and devices.