UPDATE Statement Creating NULL Fields?

I am attempting to update a table that drives a report. The report is at the order level. An order can have several types of demand (revenue $): Ship and/or Backordered. When I update one of these demand fields on the report table, I would expect those orders that have that type of demand to update, and those orders that don't have that type of demand to remain at their current value ($0 in this example which is the table default). But what is happening is that orders that don't have that type of demand are changing to NULL. What am i doing wrong with my update statement that is causing this?

Code:
CREATE TABLE #sales(
OrdNo int,
StatusGrp varchar(10),
Demand decimal(10,2)
)
INSERT INTO #sales
VALUES (1,'Ship',100.00)

[Code] .....