Calculate the difference in the average number of low-priority orders shipped by truck in each month of 1995 and 1996.
SUBTRACT(DIVIDE(SUM(l_orderkey where year(l_shipdate) = 1995), 12), DIVIDE(SUM(l_orderkey where year(l_shipdate) = 1996), 12)) where o_orderpriority = '5-LOW';
SELECT (CAST(SUM(IIF(STRFTIME('%Y', T2.l_shipdate) = 1995, 1, 0)) AS REAL) / 12) - (CAST(SUM(IIF(STRFTIME('%Y', T2.l_shipdate) = 1996, 1, 0)) AS REAL) / 12) FROM orders AS T1 INNER JOIN lineitem AS T2 ON T1.o_orderkey = T2.l_orderkey WHERE T1.o_orderpriority = '5-LOW' AND T2.l_shipmode = 'TRUCK'