recalculation V/s proof in total

You wouldn't recalculate the entire payroll - that would mean for every employee recalculating their net pay and summing all the net pays to get the payroll total. A proof "in total" makes simplifying assumptions e.g. 100 employees paid $8 - $10 an hour - so say $9, work 220 days a year for 8 hours a day - you would expect the annual cost to be in the region of 100 x $9 x 220 x 8 = $1.584m That would be compared with the expense per the G/L - if the difference is not material, it doesn't require further investigation(!)