By default, when using the decimal format tymeshift will only show 2 decimal places. Alternatively, you're using the hours/minutes, it will only show minutes and not seconds. Although the additional decimal places/seconds are not shown, they will still be taken into account when calculating metrics that rely upon each other. Thus, when you perform the calculation manually, you are not using the exact time but rather the time shown to 2 decimal places or minutes, not seconds. This is what causes the slight difference when calculating the values manually. You may change the time format to use HH:MM:SS under Admin > Settings.
When I calculate the value of a metric myself, I find that it's slightly different than the report. Why?