related: Why are there only 2 universes that are visitable?

It seems that there are differences in the universes dating back to the 1930s

• Zeppelin flight was effectively finished in the USA by the Hindenburg disaster
• Gaudí's Hotel Attraction was built over there.

Given these differences and the duration that they have existed, I would expect the two to differ much more.

The lack of heavier-than-air flight over there would have significantly altered history—the course, duration and victor of WWII in the very least. This would have global knock-on effects, imagined over 60 years. While I would expect some similarities between the universes, they seem almost identical.

Is there an in-universe(s) explanation for why there is so little deviation between the two universes? Fringe as a show is normally quite good at the science in science fiction. While I appreciate that there are plot and production reasons that the two universes have to be closely mirrored, it would seem that there is a deliberate attempt to have them mostly the same but not identical. I have not as yet seen an in-universe explanation for it. It may not have been explained yet, or I could have missed it.

As a bonus, is there a known diversion event that initially separated the two universes, or have they always existed mirroring each other?