I don't think it's the variable for counting the number of us in a group that's the issue here. There'll be some internal tracker that gives everyone in the chat group a local ID probably for the purposes of ensuring that everyone stays in sync.

If you leave the group and then go into a different chat group you'll probably have a different number in that group because the internal counter is specific to the chat, not to the user ID which will be a unique ID used across all interactions for that phone number.