Pivot Table will not sum filtered positive and negative time values together

Dear Friends

I'm in desperate need for help , as I have to deliver a report in the coming two days

I work in healthcare management and I prepared an excel sheet for the time of attendance of physicians compared to the scheduled clinic start time , then I used pivot table to segregate each type of scheduled time alone ( clinics have 4 scheduling times 9, 9:30, 17:00, 16:30) ... to calculate the average delay time compared to each type of scheduled starting time, I reviewed the formatting of the cells and everything , I also used the 1904 timing option so I can calculate negative values too , as sometime the physician will start earlier than the scheduled time, so here is the thing ... when pivot table extracts the data , they are accurate , but when it adds them together it comes up with a larger number as shown in the pivot table page in the attached excel sheet , the cells shows accurate numbers but the subtotals are extremely large numbers compared to the cells ...... and the weird thing is ..... this happen for certain clinic starting time , while the other clinic times appear accurate and calculate accurately as subtotals too ... Help plz

I also noticed that the problem is when there are both positive and negative values ......... pivot table has a problem calculating averages from positive and negative time values .... and when I manipulated the data so that no result will be negative , the calculation turns out fine...