Google Calendar uses old email address

This behaviour applies to other calendar apps on my Android phone as well.

I used to have an iBurst email address. I cannot remember if I used a calendar app on my Android phone while I was iBurst.

Years ago I switched from iBurst to ADSL and created a GMail address. I use my GMail address to log into all my Google apps, including Chrome and Google Calendar on my phone, as well as the Windows 10 Calendar app, which I have set up to use my GMail account.

For some unknown reason, all calendar events that I create go under my iBurst email address. In the Windows Calendar, in the left-hand pane, it says Microsoft account, under which it says Calendar. Enabling/disabling this has no visible effect. Below that it says GMail. Under GMail is my iBurst email address, Birthdays and Holidays in South Africa. Disabling the email address removes all events.

It is much the same on my phone.

It doesn't affect use of the calendar, but just seems weird to me.

My phone doesn't have default calendar setting. And nowhere can I find any reference to the iBurst email address except in (all) calendar apps. So I can't delete it. Can live with it.

Huawei P8 Lite. Android 6.0.