I don't think there's an answer to this one. I've never really understood it. There are many reasons why an amendment may be prudent and each could arise at a different stage of the process. One might wish to amend the resolution before the start of the debate to clarify or correct it. Equally, if it becomes apparent that the original motion will not secure a majority, it may become necessary to water it down. The standing order places the responsibility in the hands of the Chairman, presumably with the expectation that he or she will apply reason and common sense. As we know, that doesn't always happen.