AF-ON on D600 ... hit and miss. Me or the camera?

Can anyone help me understand how my D600, Nikkor 80-200mm using EXACTLY the same settings can produce this ...



and this ...



... within minutes of eachother?

Both pictures have been converted from RAW to JPG in Lightroom 4 and resized to meet DPREVIEW's posting parameters. No other post-processing has been carried out.

There two matches yesterday and I took approximately 200 pictures of each. The settings were: Manual mode; 1/1000s; f/4; Auto-ISO; Centre weighted metering; AF-C, AF ON, Release priority.

There were a couple of dozen examples of the focus being way off where I thought it should be. I realise that AF-ON takes some getting used to and actually I hope the missed focus WAS due to user-error rather than camera fault. But I'd like to identify what I did wrong in order to eliminate it from future practice.

Guidance sought and welcomed.