In the Activities Screen when I press the recalculator button (after importing the data) the number of concessions does not correspond to the actual number of concessional members in that activity.

The "recalculate" compares the amount in the enrolment screen with the full fee amount in the Activity Screen. If a match with a full fee is obtained the number of full fee enrolments is incremented, otherwise, if no match the concessional number will increase.