Easy one. Composite is a mix of the colour signal called a colourburst, along with the sync. This kind of signal has to be broken down into its essential RGB+sync in order to be displayed again, but the method used does not preserve the colour space as accurately as one that can replicate RGB+sync properly, in the way that YPbPr can. Hence it is a bit normal that your gamma or even your colour is a bit off from this when using signals such as that. S-video combines luma (brightness/contrast) and chroma (RGB colour) which is a bit of a wider conversion than cramming it all into a single signal the way composite does, so it looks nicer on both colour and brightness. Nothing is really wrong with your composite, you've just noticed how it operates differently from pure colour broadcasts.