It seems to me that Firebase User ID must stable for the same device and the same application. I can not understand why User ID changing.

First ID (from Aug 14) – this is emulator. But next 3 User ID Firebase take from the same application, from the same device and even from the same GEO location place. There is nothing something changing in my device (as login to Google), but Google changed my User ID.

What can induce this changing? How to prevent changing User ID?
Small notes, my application still in development and not signed. Can this be a reason for changing?
I made ordinary Java application and do nothing especially to changing User ID.