Annual Leave and Sick Leave Accruing Incorrectly

Annual Leave and sick leave accrues incorrectly for our employees.  We are using Reckon One online and it seemingly randomly adds random days to the employee's leave accruals.
We keept track of these accruals by spreadsheet and haven't been able to work out the differences, but I have just done a full reconciliation and discovered that days have been added to employee accruals at different stages over the year. 
It would be great to be able to work out how and why this happens!