The resolution level is the table that the results of a calculation are derived from. It specifies what is being counted, depending on the database selected, for example Customers and Orders.
With Campaign Manager a higher database level has a one to many link with a lower database level and a lower database level a many to one link with a higher database level. This means that when you change your resolution level your count may increase or decrease.
Within Campaign Manager the database levels are structured as follows, with Household being at the top of the hierarchy, and Order Details at the bottom.
This shows that a Household can contain more than one customer, each customer can make more than one order, and there will be more than one order detail for each of these orders. Therefore when changing the resolution level from Household to Order you would expect your count to increase, and when changing the resolution level from order to customer you would expect your count to decrease.