GPU Voltage lowers after a few seconds of load?

I have a 650Ti, and I'm looking to overclock it. I'm using MSI Afterbuer, and while I'm stress-testing etc. I have the logging graphs open.

Under max load, with no overvolting, the default voltage is 1075mV. However, I've noticed that the graphics card only keeps this voltage for a few seconds under moderate to heavy load, before going to 1067mV.

I can stop counteract this by increasing voltage by 12mV, so it starts at 1087mV and decreases to 1075mV, but I was wondering what's causing this temporary overvolt, and is there a way to stop it, to keep a stable max voltage?

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