Some are assembled using a partial seam. Which means you sew only part of the seam between two units until you can attach another unit to make the seam even. Here is one of many tutes I found by googling "Partial seam in quilting"
http://piecebynumber.com/partial_seams.htm
Another technique is using sashing to form fill pieces like I did on this quilt. I used a shadowbox layout so I could have varying sizes of sashing and fill pieces then I assembled the quilt in quadrants.
[ATTACH=CONFIG]407086[/ATTACH]