From what I've heard, some browsers re-interpret the image data as well and show their version of it.
When I size images for the web, I totally ignore what the dpi value is and just look at the pixel dimensions. If I make the height 700 px then the image will show on most screens without scrolling and the file size fits within the limits of nearly all sites I use. After I've resized (and saved as a new name), I apply just enough unsharp mask for it to look like the print (scanning usually softens it a little, possibly because FB just doesn't get flat enough on the glass). For here, I almost always scan the print. Since the scanner applies its own ideas of how it should look, I do usually have to use a little PS to get the scan to match the print (allowed by gallery rules).