User blog comment:Rupert Giles/Layout Changes: Breakpoints and Typography/@comment-4189499-20150520014724/@comment-5454372-20150520053757

@Imamadmad and 452: think I might have your answer: "device pixel ratios". To quote one of the posts in the link:

CSS interprets a device's resolution by the formula: device_resolution/css_pixel_ratio. For example:

Samsung Galaxy S III
 * Actual resolution: 720 x 1280
 * CSS Pixel Ratio: 2
 * Interpreted resolution: (720/2) x (1280/2) = 360 x 640

Seems like it mostly happens with mobile devices, but if you've got an A4-sized screen which is higher res than normal, I wouldn't be surprised if your browser or actual computer's factoring that in when it comes to CSS display.