Hey, I'm the official Reddit account for the Sports-Reference sites. Here's a long-winded explanation of what the deal is with these.
The confusion here is that our Offensive/Defensive Rating is a
different stat from NBA.com's, which is the version that's more widely used. We're using a stat developed by Dean Oliver in 2004 to measure, as Dean puts it,
"the number of points produced by a player per hundred total individual possessions. In other words, 'How many points is a player likely to generate when he tries?'"
(meaning unlike team stat the player has to be involved to generate stats) This is calculated largely using player stats like points, assists, etc.
The only team stats that go into the calculations are used to determine possessions or figure out
what proportion of team's numbers an individual is responsible for.
The
NBA uses the same name for their version of O/DRtg,
but it measures something different: the points a team scores/allows per 100 possessions with a player is on the floor. Here, there are no individual level stats, it's only based on team numbers with a player on the floor.
There are advantages and disadvantages to each methodology (this is a good blog post breaking those down:
http://thebrooklyngame.com/stat-corner-offensive-defensive-rating/). When we added the PbP data needed to calculate the NBA's version of O/DRtg, we didn't want to have two stats with the same name on our site, so we went with On/Off. What we call OnCourt is the team's NRtg when a player's on the floor and what we call On/Off is the difference in a team's NRtg when the player plays vs when he sits.
We try to have as much info as possible, so we carry both versions of the stat. You can see more in-depth on/off numbers for a player on their on/off page, linked to in a tab on the menu bar at the top of a player's page (
https://www.basketball-reference.com/players/j/jamesle01/on-off/2018)
TL;DR: ORtg/DRtg on
our site is an estimate of how many points a player produced/allowed per 100 possessions used/defended. It's much more
focused individual performance while what NBA.com calles ORtg/DRtg is about measuring
how an individual affects team performance.