Payroll Error

The last 2 payroll weeks I have had to amend the number of RDO hours a staff member has taken - I uploaded 8 hours and had to change it to 5 hours. 

The amount of the 5 hours changed but the Net pay and the tax did not.  The Net and Tax only "caught up" and changed to be correct if I changed the hours to more than 10.

I have attached a screen shot of the Payroll Category.

Also when the permanent staff use their RDO hours they dont accrue Annual Leave - how do I fix that?

The RDO's are in place so that when we are not able to work due to various restrictions, the staff still get paid normally and dont have to use Annual Leave.

Thanks in advance.