Why is Oracle using the index here?
Oracle also tracks the number of blocks that will be retrieved using the index values.  It may be your data is clustered by group_id.  In that case, it may make sense to use the index when retrieving more than 5% of the rows.  If the data is sufficiently clustered for values 5 and 6, then using the index may be roughly equivalent to doing full table scan of a few blocks. 

The relevant calculation will involve the number of index blocks needing to be retrieved, the number of data blocks needing to be retrieved, and the CPU required to access the data.  There are tuning parameters which adjust the relative costs of retrieving index versus data blocks.  This can change the plan that is being used.