Not so weird.
Imagine you are using Microsoft Outlook on one computer. To use Outlook to send and receive e-mail you must give it an E-mail address. Image that you and your wife use a single user account on the computer. For the computer there is only one user.
Then you configure Outlook by creating an account (you must supply your e-mail address.) Someday or other your wife wants to use Outlook to receive and send E-mail. She must add a second account to Outlook.
I have no idea what you do not to read each other's email but when you want to send an E-mail Outlook has too use one of the two accounts (the outgoing mail server of one of the two addresses) to send the E-mail. Apparently your wife's account is the default one and all outgoing messages go through it. Before you press the send button, open the account list and select the one to use for sending.
On the following screen capture see the arrowhead. Open that menu. You will see all the accounts managed by Outlook listed there. One of them is marked as the default one.
You may not be using Outlook but that should not matter, Your E-mail client may behave similarly.