You said you measured your seams. The quarter inch seam is sort of a misnomer because you don't measure the seam, you measure the resultant unit. Here's the standard way to check:
http://www.quiltingboard.com/tutoria...ce-t89997.html
For these tiny stars to come out right, you need to measure accurately, sew accurately and press accurately. Make sure that you are not pressing a tiny crease in the seam. Use a bit of spray starch or sizing. Measure as you go. After you make a flying geese unit (Flying goose!), measure the unit. If it's not exactly 2.5" by 1.5", your seams are not going to match, so you need to figure out what you need to change to make this unit exact. if it's too small, your seams are too big no matter what they measure. If it's too big, your seams are too small. Again, ignore the actual seam measurement and sew whatever width you need to make these units come out exact. I'm not a fan of how the pattern told you to make these FG units because of the bias cuts.