5D Mark III focusing with single point

I recently purchased the 5D Mk III and last weekend first used it for a wedding. Previously I had a Mk II and have always used the single central point only for focus as I found that this was the most accurate way of ensuring focus was on the exact point I wanted. Images that were unsharp were very few. So I used the same technique for the Mark III, using the 1 pt AF setting (not the Spot AF). However, I have noticed that I am getting a much higher percentage of the shots not accurately focused, and am thinking that perhaps the Mk III's 1pt is smaller than the central single point on the Mk II and that perhaps I would do better using the next larger setting, ie Expand AF Area (the central point and the four next to it straight up and down and left and right). Or could there be something wrong with the camera? I wondered if anyone else had come across this issue?
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Derek