Generally user's experience this when Album, Artist or Date field differs in the tracks. Select all the tracks for the album, and examine the values in Action Window > Tag.
These groupings are controlled by the Group By setting for the view. The default is "Album", which is defined as
Album by Artist Date
So if any of these differs, the grouping for the view will show them as two entities. Don't change the view settings to "fix" this; rather, set your tags correctly so the views work. Once you gain familiarity with how tags and views work together, then customize as you need.
Edit: See rick.ca's better response.
