Maximum row size exceeds the allowed maximum of 8060 bytes

In run Microsoft SQL Server 2014. Upon altering a table, I get a warning about the row size.

ALTER TABLE myTable
    ALTER COLUMN aRandomColumn NVARCHAR(10);

Warning: The table "myTable" has been created, but its maximum row size exceeds the allowed maximum of 8060 bytes. INSERT or UPDATE to this table will fail if the resulting row exceeds the size limit.

The table has 75 columns:

13 date
9 bit
8 int
5 decimal(18,5)
4 bigint
3 money
19 nvarchar(10)
10 nvarchar(20)
1 nvarchar(30)
1 nvarchar(40)
2 nvarchar(50)

By my calculations, the nvarchars take up 1120 bytes, the other columns 121 (not sure but it won't be much more).

How does this exceed 8060 bytes? How can I get rid of this warning?

Tried to sum all columns with isnull(datalength(myColumnName), 1) and the actual data never exceeds 600.

Found a similar question but change tracking is off for me, so it didn't help. Also cleantable, found in this question, didn't help.

However, when I copy the table, the new table does not produce this warning.

SELECT * INTO myNewTable FROM myTable;
-- (8561 row(s) affected)
ALTER TABLE myNewTable
    ATLER COLUMN aRandomColumn NVARCHAR(10);
-- Command(s) completed successfully.

Solution

Found the solution for my problem.

This question deals with the same issue, and points to the solution in another location.

This is likely due to previous alters (particularly of existing column widths) that are still reflected in the underlying page structure. Try rebuilding the table or dropping/re-creating the clustered index to reclaim that space.

What worked for me was:

ALTER TABLE myTable
    ADD CONSTRAINT pk_bigchange PRIMARY KEY (aColumnWithUniqueNonNullValues);
GO
ALTER TABLE myTable
    DROP CONSTRAINT pk_bigchange;
GO