Probably one of the most confusing and counterintuitive things that happens on my Macs takes place when I use my browser (Safari) to log into iCloud.com.
It seems that the login is asking for my Apple ID and password. Auto-filling the Apple ID address, the box to enter the password pops up. When I use my Apple ID password my request to access iCloud.com is rejected.
However, when I use my computers system username and password – as if I was logging in to my computer – I am granted access to iCloud.com.
This has been going on for quite some time. I just recently (within the last couple of months) figured out I need to use my computer’s user account and passcode.