I'm guessing it's vector vs raster. When you screen shot the image, it gets saved as a bitmap image of the screen at the time of screen cap. I bet it looks good at 100% zoom but looks fuzzy at higher magnification, right?
There is a PhotoShop trick of using Unsharp Mask, but all that really does is increase the contrast between adjacent pixels. Overly aggressive use makes the picture a lot worse. You can always increase the font size in the ebook and down sample the screen shot to the size you need.