It's like saying 'toast' when you eat toast. Or saying 'shoes' when you put shoes on. Or even saying 'sex' while having sex.
It's just a bit odd once you think about it.
Please add more examples ;)
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People say bump when there is nothing else to be said or added to the post, but they want it to stay or be relevant. As you said, the act of commenting on a thread is referred to as bumping, so therfore it is logical that that is what you type when nothing else needs to be added to the OP.