Precisely: for some reason you're not allowed to say "I assign a 70% probability to X being true" without people looking at you funny, and even "I think X is more likely than not-X, but you shouldn't be as confident of this as you are of most things I tell you" is kind of awkward, but "I believe so" is a pretty standard idiom for expressing high-probability-which-is-still-non-negligibly-different-from-1.

If you're stuck trying to communicate in an innumerate language then you use whatever phrasing you have available.