Upload & Sell: On
I suspect that it's because you're at or below the level of light that the 5D and 5DII can accurately focus.
I estimate the amount of light in the two situations as about 3 Exposure Value (EV) and 5 EV:
t 1/125s, f/1.2, ISO 1600, EV = 3
t 1/100s, f/1.2, ISO 600, EV = 5
The AF metering range for both the 5D and 5DII is shown in the manual to be -0.5 to 18 EV, using a 50/1,4 and ISO 100. To me, it figures that increasing the ISO that you're using to take photos doesn't increase AF accuracy in low light, as it simply multiplies the noise at the low levels of sensitivity. If you scale the AF metering floor with ISO (ignoring the f/1.4 vs. f/1.2 difference), you get the following equivalent EV levels for minimum AF metering:
ISO 100, EV -0.5
ISO 200, EV 0.5
ISO 400, EV 1.5
ISO 800, EV 2.5
ISO 1600, EV 3.5
So, in your first condition with EV = 3, you may have been below the AF metering range of teh camera, while in the second at EV = 5, you were within the AF metering range.
I'm not sure if this line of reasoning is correct, but it really doesn't matter to me. In these conditions, I use focus assist from a Speedlite or ST-E2.