Have you queried some when information_schema.table_constraints or sys.key_constraints to discover a primary keys in your database? And as a result from select * from information_schema.table_constraints you noticed that there are primary keys without TABLE_SCHEMA and TABLE_NAME in resultset?
You can be confused with this but the explanation is simple. Your primary keys defined in table-valued functions have been listed here. So if you have a function defined as
CREATE FUNCTION [dbo].[fnTest]
returns @resultTable table (test1 int, test2 int primary key (test1))
insert into @resultTable(test1, test2)
select 1, @foo
you will have as CONSTRAINT_NAME something like "PK__fnTest__2D27B809" and TABLE_SCHEMA and TABLE_NAME will be NULL.