This is George's answer on this problem:

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This happens because the 2nd version will not return rows where the data is null.

Declare @Temp Table(Data Int)

Insert Into @Temp Values(NULL)
Insert Into @Temp Values(1)
Insert Into @Temp Values(2)

Declare @Search Int

Set @Search = NULL

Select * From @Temp Where (@Search Is NULL Or Data = @Search)

Select * From @Temp Where Data = IsNull(@Search, Data)


The second query will return 2 rows instead of 3.