I'm guessing they are selecting groups of maybe 10 or 20 individuals at a time and will keep those remaining who passed the online bit waiting in the wings until those selected have been assessed.
If they are able to fill all posts with those already selected for interview, guess the rest of us will get a no thanks letter. If they do not fill all posts, they will select the next batch for interview.
How they select the first/ second/ third batches etc I don't know. Only my guess. Culd be completely wrong.