While on the one hand this keeps it free from spam, it also means that you may have false positives. To reduce false positives, it means managing a reasonably large contacts list.
Normally, senders on your Safe Senders list do go to your Inbox. However, there are some domains that are managed by Outlook.com that are frequently targeted for spoofing. If a message comes from that domain and it fails authentication, the safe sender is not respected. This is to prevent a spammer from spoofing one of your contacts and getting a free pass to the Inbox. Since the message is not authenticated, it goes to your Junk folder even though the sender is on your Safe Senders list. But since we can't trust the sender, and this domain is frequently spoofed, we treat it as if they aren't on your Safe Senders list.
That's why it goes to Junk.
The reason why there is confusion is because normally when a message in Outlook.com fails authentication, you'll see the red Safety Tip that the sender failed the fraud detection checks:
However, in Exclusive mode, you see the yellow safety tip about only accepting email from your Safe Senders list. There is no explanation that it failed authentication and therefore Safe Senders were not respected.
So that's why sometimes a message on your Safe Senders list still goes to the Junk folder. And, while it occurs most frequently for users with Exclusive mode, it can also occur to users in Standard mode for a safe sender if the message fails authentication.
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Now, I realize that this safety tip in Exclusive mode could give more information. So, I'm pushing internally to change the yellow safety tip about "Safe Senders only" to the red safety tip about the sender failing fraud detection checks. That should hopefully give more clarity about why the message is in the Junk Email folder.