Radius length problem

[paulco01]

Hi All.

Wondering if one of you wizards can help me out here?

Please see attached images. So basically I am creating a wooden bag with a living hinge out of 3mm ply wood.

I have calculated the length of the semicircle at the bottom and top of the design and made 5 notch cutouts at the bottom and 4 at the top. Each cut out is 4mm X 3mm deep.

When I cut out the design on my laser and assemble, I get, what looks like an over length radius on the outer cover which wraps around the end pieces, therefore not allowing the pegs to sit into the notches, as can be seen in the pictures. 

I started the "living" hinge at the center point of the radius and followed it round 180 degrees, all pegs and notches are equidistant apart. 

https://a360.co/3y10Phj to the design. 

wondering if someone can help with why this happens please?