an interesting question, it may be to allow the use of quick adjust camber adjusters such as these
http://www.rallydesign.co.uk/product...cts_id=1 1837
they require a smooth bore in the wishbone then the top ball joint is screwed into the adjuster
if this is not the case i cannot see a reason for the front upper bone boss to be so big since in its the most lightly loaded wishbone on the car
tilly