Chrome: pixel-ratio to my screen

When trying to develop a screenshot-feature for Selenium Webdriver while using os-functionality to screenshot (.NET: Graphics -> CopyFromScreen()) I recognized that chrome’s pixel aren’t the same then my monitor’s pixel (resolution: 1920 * 1080). Comparing Page Ruler’s (plugin for Chrome) pixel with Greenshot’s (Screenscraper tool for Windows) pixel, I found that there’s a ratio of 1.25. So if you have a chrome distance of 100 pixel, you got a os-distance of 125 pixel. It’s the same ratio for height and width.
The header of the Selenium driven browsers have the following heights at the moment:
Chrome: 82 px
Internet Explorer: 54 px
Firefox: 70 px