iPhone can still connect to my WiFi, even though the password was changed
If you put that password into a different Apple device then that password will be accessible to the phone over its cellular data connection, assuming that they are both using the same Apple account.
The FAQ for iCloud Keychain states

What is iCloud Keychain?
iCloud Keychain keeps your Safari website usernames and passwords, credit card information, and Wi-Fi network information up to date across all of your approved devices that are using iOS 7.0.3 or later or OS X Mavericks 10.9 or later.