explanations about colours gamma

Yes, gamma is wrong and i can notice this problem on my oled. And if you choose 10bit colors the blacks raise a little.

It's hard to say, but my first guess would be that the video quantization range indicated by the Xbox 360 is wrong. When switching to YCbCr, the quantization is implicitly limited range by virtue of the ITU-R BT recommendations, and the display ignores the
parameter indicating it.

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Given how many strange problems there are now with the HDMI behavior on the Xbox 360, it seems possible that it might output full range RGB even when levels are set to Standard. I must admit I've completely lost track by now of how the system handles levels
between playback sources and output, and if it behaves differently from time to time.

The improvement most provably has to do with the YPbPr BT.709 setting forcing the Xbox 360 to recalculate the output to limited range levels (16...235), which many tv's expect. When set to RGB and Expanded, the Xbox 360 outputs levels as full range (0...255),
which causes too high contrast and incorrect midtones unless the tv expects levels to be full range.

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Normally the source should indicate to the display whether the output is limited range or full range letting it switch to handle either correctly, but the Xbox 360 doesn't indicate it properly because of bad coding.

Which is what I figured, but wouldn't putting the Samsung in PC mode with HDMI level set to Normal mean the TV is expecting a full range?

Also using RGB and Limited did not help my picture, which should be the same.

Which is why I don't get why merely selecting YCbCr fixes all the issues where every combination of RGB doesn't?

I don't have much to offer in the way of an explanation. As a possible way to deal with it though (and probably something you've done) have you tooled around with the video levels in the game's options (gamma, brightness, etc)