Three divisions in 2021 are on pace to achieve unique feats. The AL East can potentially end up with 4 teams having 90+ wins, the NL West is almost certain to have 2 teams with 100+ wins, and the NL East is very likely to have 0 teams reach 90 wins.
AL East:
Final win projections *As of the morning of 9/24/2021 are as follows:
FiveThirtyEight, PECOTA, and TeamRankings are all projecting there to be 4 teams in the AL East with 90+ wins. (I did not include the Orioles in the chart because I am focusing on the teams in the division still with a chance to get 90 wins).
Records vs. non-division
Rays: 45-35 = .563
Red Sox: 48-35 = .578
Blue Jays: 47-36= .566
Yankees: 56-30= .651
All 4 of these AL East teams are 10 or more games over .500 against other divisions, which speaks to the strength of these teams.
MLB Teams were not separated into divisions until 1969 when they broke up into 4 divisions. Each of these 4 divisions had 6-7 teams in them so the likelihood of having 4 90-game winners was higher. In the 52 seasons from 1969 to today, there has only been 1 occurrence of 4 90-win teams in a division. This was the 1978 AL East where 4 of the 7 teams won 90+ games.
In 1993, the divisions broke up into 6 divisions instead of 4. As mentioned before, no division has had 4 teams win 90+ since then, and there have only been 8 occurrences of just 3 teams with 90+ wins.
3 teams in a division to win 90+ games
From 1993 to today, 3 teams to have 90+ wins in a division occurred just 8 times. The divisions are: 2002 NL West, 2002 AL West, 2006 Al Central, 2011 AL East, 2012 AL East, 2013 NL Central, 2015 NL Central, 2018 AL East. For each of these occurrences, the average win total for the 4th place team was 74 wins. Furthermore, no 4th place team came within 9 games of getting the division to 4 teams of 90+ wins.
You may have noticed that 2002 is the only year in which multiple divisions had 3 teams win 90+ games. Interestingly enough in that year, a team from both divisions was in the World Series (Angels from AL West, and Giants from NL West).
NL West:
Since 1993 when the divisions split into 6, there have only been 2 occurrences where multiple teams won 100+ games from the same division. This happened in 2001 (AL West), and 2018 (AL East). Barring a catastrophic final 9 games for the Dodgers or Giants, this group will have company. The Giants only need to win 1 more game to get to the 100 win mark, and the Dodgers just need 2 wins. FiveThirtyEight, PECOTA, and TeamRankings are projecting the Giants and the Dodgers to finish with about 104 wins.
Records vs. non-division
Giants: 54-32 = .628
Dodgers: 53-30 = .639
Both these teams are over 20 games above .500 vs. other divisions.
NL East Teams:
Switching to the NL East, this division will not have a 90 game-winner at all (unless the Braves win 10 of their final 11 games). Their projected win totals are as follows:
FiveThirtyEight, PECOTA, and TeamRankings are all projecting there to be no team reach 90 wins.
Records vs. non-division
Braves: 40-41 = .494
Phillies: 39-44 = .470
Mets: 38-45 = .458
Marlins: 32-51 = .386
Nationals: 34-43 = .442
As you can see from above, every team in this division is below .500 against teams outside of the division.
As the final 2 weeks of the 2021 regular season commence, it will be fascinating to keep track of how these divisional races end up.