You may not have even noticed before, but when you specify an InDesign Document size in pixels for web design, the 100% zoom level (View Actual Size) will display them larger on your monitor than it should. For example, if I create a 1400px wide document and view it on a 1920×1080 (HD) monitor, it fills the screen for some reason.
1920 is not equal to 1400
Thankfully, someone has written a script to correct this. You’ll find the scripts, and instructions on how to use them here: