Precision/Rounding when using calculate functions

[PeterB.]

Hey there,

 

I don't know if this has already been asked so sorry if I missed something.

 

I like using raster systems when designing or refining logos / graphics. In this case I started with a grid system 30mm / 5mm steps. Then i needed one 5mm grid divided into 3 parts for a border construction. 

 

So I drew a circle and divided it by 3 using the transform panel. For the second, larger circle i cloned the smaller one an multiplied it by 2 (again using the transform panel). This gave me the 3 circle sizes I wanted to use, to construct the other parts (see screen1.jpg left/right/bottom/top).

 

I cloned the circles a few times but somehow the middle sized one didn't fit to my guidelines which I set using the inspector to get exact positions (see screen2.jpg).

 

After some research I found out that the size of the middle circle was not what I thought. I should have had a diameter of 3.3333... but it actually has 3.4.

 

Can you please tell me what I did wrong?