PageSpeed Insights uses lossy compression instead of lossless. That means PageSpeed reduces the quality of your images in order to achieve the results they recommend. Instead of lossless compression, PageSpeed uses the freely available ImageMagick utility at quality 85 to compress images.
We use a slightly better setting to compress images and PageSpeed may think this is inadequate. You can manually set the compression level to Custom and select the maximum compression to ensure that PageSpeed does not flag your images.
Posted in: Image Compression