Different email for sign in to view login history?

I have both Windows 11 and 10 on my machine, although I've been running 11 exclusively for several months now.
I purchased a sub for MS 365 personal several days ago, and happened to notice that there was a 2nd device with a strange name (not the name I used for the system) listed in my devices.
My first step was to Google the cause, and was led to check my sign-in activity.
When I tried to log into sigh in history (from security) I was asked to verify that it was me who was attempting to access this info, even though I was already logged into my primary MS account.
I entered the same email I use for my MS account, but the verification code never arrived in that email box. I checked spam, but nothing. I am using gmail from my browser (Firefox).
I noted that the email hint had the first 2 characters of my email, plus the domain (gmail.com), but I have two email accounts that have the same first two characters.
This morning, I tried the other email, and the verification code arrived in that email.
What has me mystified is why MS still has this older email account, long after I switched it. I looked everywhere in my MS account info but could not find that email address.
I logged into Windows 10 to see whether I was still using it on that OS, but I only found the same info I had seen from Windows 11.

So far as the strange device name, I think it could possibly have been the default used when I installed either Windows 10 or 11. I do know that it is likely I used the old email to create my MS account, but that was several years ago.

Any thoughts?
Thanks for your help
Ultrarunner

Use of a secondary email address in that situation is to be expected. It should be listed on the "Your info" tab.

Security info confirms your identity
Security info is an alternate contact email address or phone number that you add to your account. If you forget your password, or if someone else is trying to take over your account, we send a verification code to that alternate email or phone number. When you give the code back to us, we know that you’re really you so we can help get you back into your Microsoft account.

We use your security info only to verify your identity if you're accessing personal information or if we detect a problem with your account.

Microsoft account security info & verification codes​

Click to expand...

Did Sign-in activity list the correct location of your computers?

Yes, it did. That's why I'm not really concerned.
What I am perplexed about though, is that MS still has my old email in their database, yet I don't see it anywhere where I can change/delete it.
Perhaps that old PC goes back a lot further than I thought; like before my new build, which was in 2020. I had Win 10 on the old machine, but now have only Ubuntu.