Please tell me why the following two scenarios show different results:

Scenario One:
When working with just one worksheet:
In cell A1 I have a completely blank cell with no value. In cell A2, I type
"=A1" (without quotes). The result shows in A2, naturally, another blank
cell.

Scenario Two:
When I have two worksheets Wrksht1 and Wrksht2:
In Wrksht1 in cell A1 I have a completely blank cell. Now I go over to
Wrksht2 to cell A1 and type "=Wrksht1!A1" (without quotes). It shows the
value 0.

Why is the result in A2 of Wrksht2 not showing a completely blank cell like
in scenario one? Why does it change to a zero?