This article gives a rather technical explanation, but the summary is clear "we are all living with the consequences of climate modelers stubbornly using generation after generation of models that exhibit too much surface and tropospheric warming".                   

"That’s bias, not uncertainty, and until the modelling community finds a way to fix it, the economics and policy making communities are justified in assuming future warming projections are overstated, potentially by a great deal depending on the model."