Why is it that not all users have a ‘session_tokens’ meta_key record in the usermeta table?

I’ve created my own code based on this good article – https://shocksolution.com/2019/04/16/find-last-login-time-for-wordpress-users-in-the-sql-database/ to get the last login date for a user.

I’m finding that not all users have a ‘session_tokens’ meta_key record in the usermata table despite having logged in several times.

Could some one explain that please?

Many thanks