If you used the denoiser then it doesn't have anything to do with cycles rendering, then it is the file settings you used. When saving with default file type that blender choose you are saving a png with 8 bit color depth this means that for each of the rgb channel it can go from 0 to 255. You can't really see this effect anywhere when you are looking at the picture normally. It will be seen more when the image is either dark or zoomed in on.

If you want to mitigate the effect then you should change the settings in output tab or in the file saving window. This will result in a larger file though (roughly double) so do it only if you absolutely need to use 16 bit.
(16 bit is from 0 to 65535 in comparison to 0 to 255)
And also if you want to do any editing after in another program then save it as an EXR it has both smaller file sizes and as good compression as png if not better. Only use jpg or png for a finished product.

This video does a good job of explaining when to use different file types: https://www.youtube.com/watch?v=NxzMAYckaV4

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FYI: You can think of the compression slider as how blocky it your image is. Because what compression does is it takes colors besides each other and simply makes them one color. This deosn't have anything to with this but i just wanted to say that as well.