Question:
You have read distances of 34 or 66.5cm to the danger point with 20 cm ground clearance, how can that be?
 
Answer:
There are statements in the standard about what to do if a ground clearance of 18 cm or smaller is not possible, e.g. because a vehicle has to move over ground that is too uneven. The clearances specified in this context are "not safety clearances" but minimum requirements that impede access but do not prevent it.
 
If you want to use an answer for your risk assessment, please buy the Standards research, check it, and also check very carefully if the given case really applies to your machine. If you have any doubts, feel free to contact me.
Please remember that there could be C- Standard specifically for your machine that contains statements on the subject. My statements refer to the B-Standards. If you provide me with your C-Standard, I will research your C-Standard on the subject.