The speed difference is just a difference in the exact speed that was chosen to display, the first (1 GHz) is the actual clock speed of the VRAM, the second (4GHz) is the effective clocks (effective speed being the usual one used when referring to ram speed), as the chips are QDR, meaning that the effective speed is the actual speed multiplied by four. As 4x1=4, there isn't any difference in VRAM speed between the 4 and 1 GHz ones.
It does look like a few of those have factory-overclocked memory, explaining some of the other speeds reported.
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