This is similar to my previous question, as this is unfortunately a frequent quirk in Sketchup I see.
Starting from the left in the below picture, I set a guideline 8 mm to the right. I then used this guideline to create a circle of radius 8 mm. Unfortunately, this circle is touching neither the line to the left, nor the line at the bottom, despite being of the correct radius.

Circles are made of segments, this means the vertices are slightly different to the flat edges.
By pulling your circle out on axis, meaning the tool is showing a red, blue or green axis, the vertices will hit the points you want.
This is also dependent on the number of segments you have used, if it is an odd number you wont hit both guides.