Post your part so we can see what you have modeled. Will keep us from guessing what you are doing from your work description.
Use the Advanced Editor link in the upper right of the dialog box.
Are you measuring faces or edges?
I suspect that you are not measuring the wall thickness, but the distance parallel to the parting line. That would be 1/cos(3 deg) times the wall thickness, which gives you exactly the number that you are measuring.