Truth is stranger than fiction, because fiction has to make sense. This is especially true for alternate history. (How realistic is OTL? Despite the total absence of aliens, espers, sliders, time travelers, or a surviving CSA, an astonishing number of people say "ASB.")
A lot of world history is, in fact, pretty improbable. Is it implausible? Hell no: it actually happened at least once, and is well documented too. Despite this, many criticisms of plausibility tie directly into how "probable" something appears to be.
So what is probability, anyways?
...a pretty chancy thing.
- All numbers are equally unlikely to come up. If you roll a 100-sided die, you have only a 1% chance of getting a 1 - but you also have a 1% chance of getting any other number.
...something that takes setup.
And an author's execution of that setup can make or break their work.