Yeah I kind of got into it above responding to Squally, but more or less there are two components
1. Playoff Path: How likely are you to make the playoffs, and if you do make the playoffs how likely are you to get a bye? Cubs are more or less locked into that #4 seed. So very simplistically a team that locks up the four seed has odds that are 100% of making the playoffs times 50% of surviving the WC round times 50% for the LDS, ditto the NLCS and WS. So 1*.5*.5*.5*5 = 6.25%
2. Team quality: This is where things can get messy, and where the Mets/Cubs split is likely coming from. Fangraphs has a live depth chart for every team, here is the Cubs:
https://www.fangraphs.com/depthcharts.aspx?position=ALL&teamid=17
These depth charts take projected WAR for each player prorated towards playing time that is manually adjusted by a dude who works at FG (Jon Becker). This projected WAR goes into a math blender along with strength of schedule to get a projected winning percentage. And that win % goes then into a different blender to adjust those playoff round %s from being 50/50 by default to like 60/40 or whatever. A team that's 50/50 in each playoff round is 6.25% to win it all like I had above. A team that's only 40% in each round would have 2.6% world series odds.
The Cubs' current ROS winning % is only .512 (83 win pace) while the Mets is .588 (95 win pace). A big part of this is SOS, .504 for the Cubs and .488 for the Mets. Another part of that is projected WAR. This doesn't have one culprit but look at that depth chart, a lot of choices on that page are extremely reasonable for the regular season but become ludicrous for the postseason, such as Colin Rea's prominent playing time or Kyle Tucker's lack of playing time.
The FG continually fiddles with these depth charts, at this late stage those changes can have huge impacts on the World Series odds because of the chaining effect of the playoffs (the 50/50 vs. 60/40 deal). They'll do a big pass at the end of the regular season and you'll see the numbers look a lot more reasonable (i.e. flatter across the league) from that point onward.