PS/2 vs USB Setup
How is this possible? this dedicated keyboard port uses what is called a hardware interrupt or cpu interrupt. What this means is as soon as the key is pressed it goes directly to the cpu for processing.
USB on the other hand uses a method called polling. By default on windows a usb based keyboard will run at a frequency of 125hz and some keyboards can come with 1000hz and you can manually change the default value if you wanted to. What polling does is scan a certain number of times per second waiting for a key-press. This can lead to missed key presses as well as no support for N Key Rollover. Some manufacturers claim to sell USB keyboards that fix this problem, but most low end keyboards fail the test.