The ½ full keg would have had more contact surface area and less distance to diffuse through to get to the botton of the keg. It should have carbed first,,,, if the headspace contained as high percent CO2 as the full keg. My guess is that it did not, and possibly by far. I think in these situations it is purge, purge, purge,,,,, and when you think you are done, purge again. CO2 is cheap. Replacing spoiled beer..... not so cheap.