How does this Monitors 75hz work?

Apr 16, 2018
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To my knowledge you can only get more than 60hz is through a displayport cable/input.

That isn't true. There are certain older monitors (such as the VG248QE) that are limited to 60 Hz over HDMI, but this is a limitation of those particular monitors, not a limitation of the HDMI standard. People just misinterpreted it as a limit of HDMI itself, but it isn't.

HDMI will work at 1080p 75 Hz just fine on that monitor.