Why are these SQL Server statistics outdated, when it's set to auto-update?

We have a SQL Server 2012 instance, with auto-statitics set to ON for the DB:

But then i ran a query to check some statistics, and some haven't been updated in a while:

1. Why is this the case? Is there a rule to whether SQL Server updates statistics that haven't been triggered by these indexes?
2. Do i need to care? How do i know if i need to update them, or if they are causing performance issues for me?

Thanks!