Images in Browsers

Does anyone know why sometimes when you right click on an image in a browser (IE, FF or Chrome), and save the image on your hard drive, you get a different file size image and lower quality than the original image you uploaded to the server? This happens even if you clear the browser cache.

What is strange is that it doesn't happen all the time. What is stranger is that I wrote a simple html page with a link to the image. I right clicked on the link and saved the target image. It saved it with the original size and quality. However, a little later, I tried saving the same image again, the same exact way, I got the lower quality image with the reduced file size.

I know it's not an issue with my PC because the same thing happens on my phone (Droid X) browser. When I save an image into its memory from the browser, it is lower quality and reduced file size.

The lower quality image file size is usually a little bigger than half of the original image file size.

What is going on?

UPDATE AND ANSWER:

My problem was caused by Verizon Wireless compressing images through its network: http://www.macobserver.com/tmo/article/verizon_starts_data_throttling_content_optimization/

I was occasionally tethering using a MiFi device and was apparently in the top 5% of bandwidth users. Therefore, images downloaded through the MiFi into my laptop and on my Droid X were being compressed through the network. The browser was caching the "bad" images, so they appeared compressed even when I was on a different network, making it harder to for me to troubleshoot what the heck was going on. I hope my answer helps others.

I don't want to get credit for my own answer, so I am changing this to a different question: Since a lot of people use such wireless networks now on their mobile phones and through tethering, should web developers test their sites for such use to account for the image loss? Sometimes the image loss is considerable. If so, how do we do it if this occurs only during certain periods of the month for only certain users? What compression algorithm are they using? Can we emulate the compression?

My problem was caused by Verizon Wireless compressing images through its network (see the edit to my question). This problem no longer exists with newer devices (or perhaps Verizon has abandoned this practice).