Check the KeyColumn property of the attribute GL Year, if the value of the Key is equal, they'd be automatically grouped. What you see in the list is the NameColumn property.
More info on MSDN and on MSSQLTips.
Update:
For KeyColumns property, MSDN states:
Contains the column or columns that represent the key for the attribute, which is the column in the underlying relational table in the data source view to which the attribute is bound.
This means whenever the value of the column(s) specified in this property are equal in the underlying table for any number of rows, SSAS will treat all these rows as one Member.
In your example, say if you have a column GLYear in your relational table and in KeyColumns property you have specified the same column, the resultant list in your screenshot will have a single value for each year. Hence, all rows with value 2015 will be combined together to form a single member in that list.
