This is likely due to the so-called “fuzz factor”. From the manual:
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.
Also see Possible to determine when exactly a card will be due? (Non-obvious answer、I presume)