It’s very simple. The Celtics are one of the best regular-season teams in NBA history. Their winning percentage ranks just 20th all-time, but their point differential (plus-11.6 per game) ranks fifth. Only four of their 16 losses have not been within five points in the last five minutes, with the last of those not-close defeats having come more than two months ago (Feb. 1). They were without a starter in two of the four and playing the fifth game in seven days for another.
The Celtics rank in the top two on both ends of the floor and they’ve been 5.2 points per 100 possessions better than the next best team, which would easily be the biggest such differential in the 28 seasons for which we have play-by-play data.
From this week's NBA.com's Power Rankings: https://www.nba.com/news/power-rankings-2023-24-week-25