Truth.
Really though, you saved your image as a JPEG. That's the biggest mistake you can make in Photoshop(usually).
From Wikipedia, "The compression method is usually lossy, meaning that some original image information is lost and cannot be restored, possibly affecting image quality. There is an optional lossless mode defined in the JPEG standard; however, that mode is not widely supported in products."
Also from Wikipedia:
http://upload.wikimedia.org/wikipedia/commons/c/ce/Quality_comparison_jpg_vs_saveforweb.jpg