The score changes for me too when I check the report link you shared. I saw it at 9/10 and then I refreshed the page and it went to 7/10. I don't know why it's happening but figured I'd pop a note here so you know it's not only you seeing this happen!
It seems the content of the email message you sent was basically just "testing", and this can cause a lower score because of the message contents. You should have better luck getting 10/10 with a message that looks real. If you send out a regular newsletter or something, then try using some text from one of those. It doesn't have to be a long message, I think I've gotten 10s with just a quick pretend "reminder of tomorrow night's shopping plans and a list of items to buy" or something like that, that looks like a genuine non-test email.
I can't remember the limit per day that we can use the mail-tester site, but I remember I got limited because I tried too often, so my suggestion would be to wait for more help with that -3 section first, and then if you're asked to send another test email, make sure it has a real-looking message inside it as well.