When you make a modification to an employee's payroll and give an addition (like a bonus), that leads to an increase in the employee's TDS. The entire additional TDS liability because of that bonus is included in the same month itself, so that the employee received a steady in-hand pay in the months following the bonus.
For eg, let's say an employee's earnings in a year are expected to be Rs 12,00,000. The total TDS for the entire year would be Rs 1,79,400 and it would be distributed in equal amounts, leading to an in-hand pay of Rs 85,050 (and TDS of Rs 14,950) . Now let's say a bonus of Rs 1,00,000 is given to the employee. So the total earnings would be Rs 13,00,000, and the total TDS on this would be Rs 2,10,600 (in increase of Rs 31,200). This additional liability of Rs 31,200 would be included in the same month as the bonus, and the employee's in-hand salary would be 2,00,000-(14,950 + 31,200) = Rs 1,53,850.
The reason for doing this is quite simple. If we do not deduct the entire additional TDS liability, then the employee's in-hand salary for all the remaining months of the year will dip substantially, since they will be paying more TDS every month even though the bonus was only given for one month.