sql server - ISNULL erroneous return? -

i have following query:

select apps.field4,        isnull(apps.field4, '-1')   applications apps  apps.object_id = 1727847        , isnull(apps.field4, -1) = -1  

apps.field4 integer, , no record has value less 0 field4.

the return values above query are:

+------+----+ | null | -1 | +------+----+ 

but if add and apps.field4 null clause, no records returned:

select apps.field4,        isnull(apps.field4, '-1')   applications apps  apps.object_id = 1727847        , apps.field4 null        , isnull(apps.field4, -1) = -1  

why isnull seem correctly identify null value, is null not?

i know i'm not answering question. however, found interesting in msdn :

c. testing null in where clause not use isnull find null values. use is null instead. following example finds products have null in weight column. note space between is , null.

use adventureworks2012; go select name, weight production.product weight null; go 

https://msdn.microsoft.com/en-us/library/ms184325.aspx