I just added a 960px <hr> to this web page. On my monitor, attached to a MBP running OS 10.6, it measures about 9.5 inches (by holding up a ruler to the screen). That would indicate OS X treats my screen as about 100 dpi.
The monitor is 30" diagonal, 2560x1600. So, calculating the dpi:
Yep, that's because it's 960px in CSS pixels, which are at 96dpi. Your monitor also has it's own physical dpi so you can't just measure the length of a line on the screen to infer the OS's dpi.
I just added a 960px <hr> to this web page. On my monitor, attached to a MBP running OS 10.6, it measures about 9.5 inches (by holding up a ruler to the screen). That would indicate OS X treats my screen as about 100 dpi.
The monitor is 30" diagonal, 2560x1600. So, calculating the dpi:
One of these days, I'm going to get around to buying a new Retina MBP. Then, I'll try that exercise again.