Why did the remaining money go to another address?
What you are observing is a change address.

The wallet created a new address for itself to send the change of spending the 1 BTC to. This is so that it is non-obvious to the rest of the world which is which. The 0.9999 BTC is still hers to spend, just assigned to a different address.

And it is working: if you didn't know she was sending to her brother or his address, you would not be able to infer anymore how many BTC she holds.


  I never had a reason to use the Blockchain Explorer


You generally never have to. The problem is that explorers do not know your wallet. They present a lower-level view of what is going on in the system than your wallet does, which can be confusing.

Read more: Why does Bitcoin send the &quot;change&quot; to a different address?