Different film speeds in different developers

I am trying to decide what developer to use for Ilford Delta 400 Pro (new version). I have noticed that development times are given for a variety of speeds in a variety of developers (as usual). However, in some developers, no information is given for ISO 400 - the slowest speed shown for these developers being ISO 500. Any idea why this is? The film is effectively faster when used with these developers? If so, will the grain be any bigger as a result?

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