I have both, and have noted the same thing. Also, the 5DII will only focus on center focus point in darker situations, while many more on the III. I also found side by side, the 5DII will fail to achieve focus while the III has no problem. That is not to mention focusing on moving objects.
I had a 5D2 for years and never really had too many complaints about the AF at dark wedding receptions. I ended up selling my 5D2 before my 5D3 arrived so I didn't have a chance to do a side by side comparison. The 5D3 is great and I am happy with the AF but I never really thought it was that much better at low light AF than the 5D2. Well I shot a wedding with another photographer who has a 5D2 this past weekend and just for the fun of it I took a few shots and compared the cameras side by side and I was amazed at how much faster the 5D3 was. I could get 5 good shots in the time it took the 5D2 to get focused and take one. It seemed like night and day to me. I am sure this isn't news to anyone but I thought I would share just for the fun of it.
Cameras: 1DXII,5D III, 5D II, EOS M Lenses: 24-70 2.8 II IS, 24-105 f4L, 70-200 f4L IS, 70-200 f2.8L IS II, EF 400 5.6L, 300 2.8 IS II, Samyang 14 mm 2.8 Flashes: 600EX-RT X 2, ST-E3-RT
Plus lots of stuff that just didn't work for me