Every word in your sets sits in one of three states. New means it has never been shown to you. Due means its scheduled review date has arrived. Weak means you have answered it wrongly at some point and have not yet driven it to full mastery. Due and weak both count as review, which is why a word can turn up before its date.
New
A word with no history at all. Nothing has been recorded against it, so there is no mastery, no review date and no record of a wrong answer. New words are served in the order they appear in the set rather than shuffled, so an imported list keeps whatever grouping you gave it.
Due
A word that has a review date and that date has passed. This is the ordinary spaced-repetition case: you got it right, it was scheduled a few days out, and the few days are up.
Overdue words are not penalised, they just sort earlier. Within the review group, words are ordered by lowest mastery first and then by how overdue they are, so a week away from the app does not scramble anything — it simply means the front of your next session is the words that waited longest.
Weak, and why it is the interesting one
A word is weak if it has ever been answered wrongly and its mastery is still below 5. Both halves matter. The first wrong answer flags it, and the flag only clears when the word reaches full mastery — not when you get it right once, and not after any amount of time.
The consequence is that a weak word is eligible for review whether or not its date has come round. A word you fumbled last Tuesday and have half-recovered since will keep appearing, because a single wrong answer in its history is evidence that the schedule alone should not be trusted with it.
This is the mechanism behind the feeling that the app keeps showing you the same handful of awkward words. It does, and it will keep doing it until each of them reaches mastery 5.
How the three get mixed into a session
Roughly half of each round is review, meaning due or weak words, hardest first. About a third is new. The remainder is a refresh of words you have seen and are not struggling with, least recently seen first.
If one group is short the others fill the gap, so a session never runs thin. Clear all your due words and the session simply takes on more new ones.