I was reading the paper "Consistent Individualized Feature Attribution for Tree Ensembles" by Scott Lundberg et al. and cannot understand how the calculation for the $R^2$ works here - see explanation on the image.
From this paper, the authors calculate an $R^2$ value based on the "proportion of model output variance explained". I fully understand how $R^2$ works regarding regression, but I'm not sure what the calculation looks like when used in this context.
Could someone explain to me what this calculation looks like?
