The final round of white triangles are made a little extra large on purpose. This allows the colored square to float rather than come to a point exactly at the seam line. Just one of many common quilting techniques. This block is a lovely variation of a "Square in a Square" block. It might now look simple is the form your received it but take a big breath. If you are part of a sewing group or guild or just have a quilting friend, they can give you some tips in person to help make this block perfect.
I will give a first tip: This block has a lot of bias edges, Bias edges will stretch and distort if we are not careful. Press gently and square up the block after each round when the block becomes a square again.. It will become second nature to press and square up. The final result will be a block that is pretty near perfect and flat.
In the end you will likely love this quilt so much you will not want to give it back!
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