It's similar how a printer spools documents so if your sending multiple emails they queue up in the the outbox. The process is essentially like this
Once the server acknowledges the request & authenticates said request
Data is sent to the mail server from client (your tablet)
Once the data gets sent completely it is moved to your sent items.
Basically the out box is the digital version of the out-tray (mail ready to be sent out) and the inbox the digital version of the in-tray (received mail).
In laymen terms, this how the outlook desktop app behaves and therefore the app mimics this behavior thus is completely normal.