Interesting.
Glad I'm aware of this feature now!
Let's say I have a $20 bill in my wallet. I want to give you $10 and keep $10 for myself. I can't simply cut the $20 bill in half - I must break it down in to two $10 bills, send one to you, and then send the other back to myself. This is the essence of "change addresses". When you spend some bitcoin, you can only spend bitcoin in the discrete chunks in which you received it. Any excess from that chunk you don't sent to the other party will be sent back to you. It is sent to a different unused address in your wallet, rather than back to the exact same address, as it is better for both privacy and security.
The addresses you looked up and found to be empty on block explorers will be addresses you sent coins from. You will see that they have sent coins to wherever you told them to, and they will have also sent some coins to another address - this will be your change address. If you add up the coins in these change addresses, they should match the number of coins that your Trezor wallet says you have available.
Thanks a lot for the explanation.