It actually makes alot of sence if you think about it..
What you want to measure is the CIRCLE in which the bolts are placed.
If there is 4,6 or 8 bolt holes then you measure center to center as they are placed directly opposite of each other.
If you have 5 bolt holes they are not placed directly opposite of each other and to get the circle measure you have to measure center to outer edge like in the drawing. Kind of confusing but logical in the end :)
Sorry if my explaining isn't the best, kind of hard to be technical in a foreign language..
