I don't know how you took the screenshot, but it's because of the storage format. For example, the pixels saved when you take a screenshot are too low. Look at the pixels in your picture, it should be 256 colors.

If you use software screenshots, it is recommended to save them in JPG format with good compression ratio. Try not to use GIF. Generally, the pixels of GIF are mostly 256 colors (of course, it can be higher), depending on the settings when you save the screenshot.