I’ve recently stumbled on an issue caused by Fuzz Factor not being mentioned in the manual and had to seek help here. I suggest the manual to include this information.
It’s mentioned here (though without the phrase “fuzz factor”):
Anki additionally adds a small amount of random variation to the next due times, in order to prevent cards that were introduced together and always rated the same from always staying next to each other. This variation is not shown on the time estimates but will be applied after selecting the button.