Their numbers assume static rosters and do not evaluate things like:
- Injuries, there are always injuries each year that wildly swing projections.
- Player movement - you can't predict this, and it changes a team's outlook during the season
- Post all-star break team goals. A lot of bad teams win more games at the beginning of the season when playoffs are still in reach. Once teams start facing the reality of missing the playoffs there is a shift in strategy for many teams (tanking).
- Schedule plays a big part in overall records by the end of the year. Playing OKC at the beginning of the year versus end of the year is a completely different matchup.