How about because it is a "User selectable option".

wrote:

Hi all,

I have a number of columns which are linked to column A; specifically,
the values are linked to be the same (C2 = A2, etc). Why on earth does
Excel return a 0 when the source cell is, in fact, blank?
This is so wrong! Hasn't anyone explained to Bill Gates that zero and
null are two different things?
How do I get round Excel's error? So far the only solution I found is
using an If formula (=if(a2="","",a2) , but that's like killing a
mosquito with a nuclear bomb!
Any help would be greatly appreciated!
Thanks!