When you dimension the whole arc, the ext lines are perpendicular to the arc. When you use the Partial option and pick 2 points on the arc, the ext lines are not perpendicular. However, when you pict these two points on the ends of the arc, tha lines are again perpendicular.
It's probably a bug that can be worked around by spliting the arc, but it's very uncomfortable. Considering that there's the option to dimension just a part of an arc, this should be fixed.
edit: the ext lines of the partial dimension are parallel to the line conecting center of the arc and middle of the arc part that is dimensioned