Explain the superior PSNR of PSNR anchoring at low noise

Analyze why PSNR anchoring achieves higher PSNR than Real-ESRGAN for inputs with Gaussian noise of standard deviation 25, thereby clarifying the observed performance behavior.

Background

The paper evaluates fidelity-constrained anchoring of black-box denoiser outputs by linearly blending each denoised image with its noisy input. The experiments compare PSNR-based and SSIM-based anchoring across several Gaussian noise levels and measure both denoising performance and residual-noise excess kurtosis.

For the Gaussian noise case with standard deviation 25, the authors observe that PSNR anchoring can produce a higher PSNR than the unanchored Real-ESRGAN output. They explicitly defer a detailed analysis of this unexpected behavior, leaving its cause unresolved.