Its intrinsic, I guess he takes a base line considers the inflation and deflates everyone by a mean percentage. It gets complicated. If you look at Chess where players haven't really improved dramatically, as in football, fitness, diet etc not such a big influence, from the top 20 ever ELO ratings, only 3 players feature from pre 2000. Kasparov July 1999, Fischer April 72 and obviously Karpov who amazingly creeps in only at 20th in 1994. With the greatest respect to Anish Giri who is at 13th with an ELO of 2798, it would be churlish to claim he was a better player than Karpov, Alekhine, Fischer, Capablanca, Botvinnik, Even Steinitz and Morphy could be argued to be better players had they had access to modern tools and the advantage of modern opening theory.
They now have developed other methods to attempt to compare players from different eras and the results are very different. Most of these systems though give far higher ratings to players from the past.
Its interesting though.