A couple of lines during the episode imply that there's a period of determination between death and reincarnation or entering an afterlife:
Quote:
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Originally Posted by Otonashi, to Hinata
I was able to find fulfillment in life at the end. But I lost that memory, and mistakenly wandered into this world.
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Quote:
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Originally Posted by Kanade, to Otonashi
My one regret was being unable to say thank you to the person who saved my life. I wanted to tell you so badly that I got lot and wandered here.
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So it's possible that Otonashi showed up in this world later because, without much of his memory, he could not figure out where to go and got lost "in between", only stumbling into the high school by sheer chance. Kanade, also being lost, simply happened to show up earlier, perhaps because she eventually realized that her regret was holding her back.