Grand Totals are simply a total of the numbers in the column above.  Actual Counts are real counts of the records.

For example: Let's say you have an Annual Campaign and a Capital Campaign and a single constituent gives a separate gift to each campaign. The Grand Total would be 2, by simply adding the 1 from each campaign. However, the Actual Count would be 1, because it is same constituent that gave to each of the campaigns.

 No. DonorsNo. Gifts
Annual Campaign11
Capital Campaign11
Grand Total22
Actual Count12

Note: If a gift was entered as one split gift rather than two separate gifts, the No. Donors and No. Gifts columns would be the same for Grand Totals. However, the Actual Count for the No. Gifts column becomes one because a split gift is considered one gift, regardless of how many splits it has.