On Chessable, every move you learn has its own timer and will become due for review after a certain time (see the linked article below). This means that in variations that give you trouble, some moves will be on a different timing than all of the others in the variation when you get all of the moves right except for one. The correct moves will move on to the next level while the incorrect move will fall back to level one and will be due for review after 4 hours.

Chessable prompts you to review the incorrect move when it becomes due. 'Review: Whole Variation' enables you to review the whole variation, so you will 'overstudy' the moves you got correct earlier and won't receive any points for the overall leaderboard for these moves, although you still get points for the course's leaderboard. It also means that you have to spend more time because you're reviewing 10 moves instead of just one.
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This is where 'Review: Randomized' is a useful alternative. With 'Randomized' enabled, you only review moves that are due with no overstudy whatsoever, but then you have to play the moves without the context of the previous moves, i.e. in random positions.