By using the functionality "compare dates", I realized, that
Sitecatalyst compares only items, which are available in both periods.
In case one item is not existent in one of the compared periods, it
doesn't appear. It will only be counted in the sum. I attached an
example, where we compared the orders of a productgroup with 6 tariffs
in March. In February, we had just 5 tariffs. In case I compare February
and March, the tariff on position 6 doesn't appear, because there are
only orders in March...