Why aren't my quilts ever the right size?

My sister in law is getting married this September and I wanted to make her a quilt. My hubby wants to give his little sis cash for their gift so I decided I would make the quilt for her Wedding Shower. That gave me a months less time to procrastinate so I got started. I figure if I can have it done by this Friday I can take it to get quilted and the turn around is about three weeks and that would give me a week so sew on the binding. So here is my start.

Is it loud enough? I was just fascinated with the Coats and Clark advertisement that featured this quilt. Its Kaffe Fassett's Hot Diamonds.

But I am not one to have this much color in my life. I like color but as you can see this is a lot of color. My Sister in Law and her fiance are not afraid of color. She's an interior designer and he does some sort of design . He creates chandeliers and light fixtures for some of the rich and famous. Anyway, they have an orange wall in their living room so I figured this would be a great lap quilt. So, here is my problem. I sewed everything together and according to the template if I made the large diamond, 144 pieces, 12 rows up, 12 down the quilt would be like 54x90 inches (that's a lap quilt to me). Some how the quilt is only like 59x56. I don't know how it happened. I swear I used every piece and I even had extra that I used. So, I am going to add a 15 inch border and make it a Queen Size quilt. I can never make my quilts the right size. Oh well. When I am all done I will take a completed picture to share. I like it but it sure is LOUD! I hope she likes it.

Oh and Diamonds are not that easy. Way too much brain power needed. You can't just line the seams up and sew. It has to be off slightly so it lines up right. Luckily, I figured it out on my third row of where the placement needed to be. I think I might have to make myself a diamond quilt now.