You have mentioned that you know this means that the flag was declined based on voting in the review queue. It is possible to find even specific review in question. You can find it either manually looking at review queue history. Probably more efficient way is to use this SEDE query. However, since data in data explorer are only updated once a week, usually you have to wait some time before you can find the review in this way.
But I think you do not learn much form the review, only how many users agreed with you or disagreed with you. Probably the only relevant information is whether the post already went through the review queue, but this is only additional information in the case of close votes - since this was a close flag, you have already seen that it was declined. (Looking at review is more useful in the case of suggested edits, where the users who rejected the suggested edit also provide the reason why they chose this option. But for suggested edits it is easier to find the review from your profile; they are listed in the activity tab among suggestions.)
You also ask why the users chose the option to leave open. We can only speculate, but there were some suggestions in comment what the reason might have been:
- One explanation might be that they simply did not check the proposed closure carefully enough.
- Another possible factor might be that both questions are tagged proof-verification. This is a special class for questions, where the OP does not ask for any proof of the given statement but for critique and possible improvements of their own proof. There were many discussions on meta about this type of question, you can check meta posts tagged solution-verification if you want to learn more. In the case of closures, the consensus seems to be that such questions should only be closed as duplicates after the OP received sufficient feedback on their own attempt.
- Another possibility that user who were burned too many times by review-audits are extra careful. So they might open the post in new window to see whether it might be an audit. In this case if a user opened the post in new window, they saw that there are no close votes. (The reason is that it was flagged, not close-voted). For this reason they might have been suspicious that it is a review audit, so they have decided to stay on the safe side and vote to leave open.