It's been that way as far back as I can remember.
If you don't want to look at them separately, don't look at the individual ones at the bottom.
Move your cursor over the ones at the bottom and you'll find the option to either Hide or Show them.
Everything can be seen combined at the top under Messages.
Could be, you are only just now discovering the multiple options you have for viewing your email with the Mail app.
Don't like those options, don't use them.
They are not assigned at random, they are assigned based on which email address you select to use when sending them.
You can also select any sent email in the list - hover your cursor over your name at the top of the email which will cause it to highlight and an arrow will appear.
Click on that arrow and you will find a list of the email addresses you have used with a check next to the one used to send that particular email.