There is a difference between "not acting on a close vote" and voting to "Leave Open" a question. Not acting (or choosing "Skip")
removes the post in question from your review list, while voting to "Leave Open" has an impact on the review list displayed for other users (read further).
You are able to view the previously reviewed close-votes by clicking on the "history" tab under Close Votes:
In terms of the difference between voting to "Leave Open", the following is taken from How about a "Vote not to close" option to counter the "Vote to close"?:
- Voting against closing does not override anyone's close vote. However, a sufficient number of "Do Not Close" responses (currently 3) will kick the question out of the review queue and start aging the close votes - regardless of how many views the question has had.
- If the question is closed, "Do Not Close" votes do not translate into re-open votes. However, [it may be used] to prioritize items in the Reopen Queue [..].