Trusted Senders are the "envelope sender" of your message. The address you see in your email program is the "From: address". In many cases, the envelope sender and the From: address are the same. But they don't have to be!
The most common reason why the envelope sender won't match the From: address is, it's being sent on someone else's behalf. That's almost always because it's a listserve or an email marketing message. The envelope sender is where any errors would be sent (like a bounce message). The From: address is where you, an actual person, would send an email. The message sender wants those to be two different places because, if you have more than a few messages, the person who should be in charge of answering actual emails could very quickly become overwhelmed dealing with errors and bounce messages.
Let's look at an example. So, I got a message the other day from J.Crew. The From: address was:
Great, I totally recognize that as a legitimate address, and, if I responded to that message, it would go straight to J.Crew customer service. The envelope sender, though, was
And that's the address I would find in the Trusted Sender section.
When you release a message from the Spam section, we automatically create a Trusted Sender entry for the envelope sender. So, even though you think you're releasing a message from firstname.lastname@example.org, behind the scenes, we're adding bo-12cqh.....@b.e.jcrew.com for you. So, that's why you may see addresses in the Trusted Sender section that you don't recognize. And that's also why we recommend that you never delete entries from your Trusted Senders!