Many out of gamut colors are clipped with Perceptual too, there's no way around this! You've got a lot of out of gamut colors, hence they can't fit within the new color space. But with RelCol only the out of gamut colors get clipped to the boundaries of the new color space, in gamut colors are left alone. With Perceptual, other colors can be mapped into the new gamut but those out of gamut colors will still get clipped somewhere.