Excel iterate over table and list output with spilling formula

MichielS115 An alternative lambda-free method (non-iterative) could be:

=LET(
    table; A1:O7;
    col_labels; DROP(TAKE(table; 1);; 1);
    row_labels; TAKE(DROP(table; 1);; 1);
    values;     DROP(table; 1; 1);
    net_change; values - HSTACK(TAKE(values;; 1); DROP(values;; -1));
    VSTACK(
        HSTACK("Item"; "Date"; "Net Change");
        HSTACK(
            TOCOL(IF(SEQUENCE(; COLUMNS(values)); row_labels));
            TOCOL(IF(SEQUENCE(ROWS(values)); col_labels));
            TOCOL(net_change)
        )
    )
)

Adjust the table range reference as needed.

Also, if you prefer blanks instead of zeros for the first month, adjust the net_change variable as follows:

=LET(
    ...
    net_change; IFNA(HSTACK(""; DROP(values;; 1) - DROP(values;; -1)); "");
    ...
)

See attached...

MichielS115 

My preferred approach was to offset the data and subtract before unpivoting.

= LET(
    offsetData,    HSTACK(TAKE(data, , 1), DROP(data, , -1)),
    difference,    data - offsetData,
    broadcastDate, IF(ISNUMBER(data), date),
    rowHeader,     IF(ISNUMBER(data), headings),
    HSTACK(TOCOL(rowHeader), TOCOL(broadcastDate), TOCOL(difference))
  )