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Most mechanical keyboards have N-Key Rollover or Anti-Ghosting to handle the situation of multiple simultaneous key-presses. USB supports up to ten keys, while only PS/2 has full N-Key Rollover support.

Here's a good explanation of the issue with an interactive typing test showing the issue: http://blog.controlspace.org/2010/08/n-key-rollover-what-it-...

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USB keyboards can get around the ten key limitation by presenting themselves as multiple devices.

Or by using a passive USB to PS/2 adapter. USB and PS/2 are both serial protocols with similar wiring, so a USB keyboard that supports passive adapters can detect what kind of port it is plugged into when powered on, then send unmodified PS/2 through the USB cable. For this reason, there are a few mechanical keyboards that only support N-key rollover in PS/2 mode, dropping down to 10-key rollover in USB-HID mode (it's cheaper and less likely to cause bugs than presenting as multiple HIDs, and anyone that cares about N-key rollover probably has a PS/2 port anyway).

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