https://en.wikipedia.org/wiki/Glicko_rating_system
---------- Post added at 11:23 AM ---------- Previous post was at 11:17 AM ----------
Most chess federations don't use the plain ELO system anymore. They use improved algorithms like Glicko. Glicko is ELO but adds an RD (ratings deviation.) For example, two people can both have an ELO of 1700, but they get different RD's based on the number of total games they have played to reach that ELO (experience) as well as the ELO and RD of their opponents. So a 1700 vs 1700 won't necessarily be a +15/-15 scenario, but one may stand to gain or lose more points than the other because of their RD. Someone who has reached 1700 by beating noobs would lose a lot more points against an opponent who is more solidly 1700.