One way for this to happen (and frankly I don't know of any other ways) is if you keep on creating new accounts when you "change" your email address, instead of actually changing the address on the same account.
Here's how you change the email address (Apple ID) you use for your Apple Account while retaining everything in your account: Change your Apple ID - Apple Support :
- If you don’t have an iPhone or iPad the support article says to, “go to https://appleid.apple.com and sign in. Then follow the onscreen instructions to change the email address associated with your Apple ID.”
- For general information, and other issues such as changes or addresses not being accepted.
- Note a recent change to the article where you can now change the email address without having to have access to your old email account.