Avg of DateDiff Formula

I created a formula to get the minutes between 2 times using DateDiff:

datediff ("n",{PAT_ENC.AVS_PRINT_TM},{PAT_ENC_HSP.HOSP_DISCH_TIME})

This formula is displaying the correct amount in minutes for each patient but my average is way off. In the example below I have 4 patients where the average should be 64.25 but my report says 40.19. I am using a summary to get the average of the DateDiff field. Not sure why it is not giving me the correct average.