Why did my units reduce after the switch order is complete?

The reduction in units is due to the difference in NAV of the two funds. Suppose the NAV of the switch in the fund is higher than the switch out fund (Note that: Investing Amount remains the same)then:

NAV X UNITS = Amount 

Since the amount remains constant/same, higher NAV will result in lesser units leading to a decrease in the number of units of switch in the fund.