($3200 per month)*(12 months) = $38,400
There are 52 weeks in a year, so if you pay every 2 weeks, you make 52/2 = 26 payments:
($1600 per payment)*(26 payments) = $41,600
The difference is:
$41,600 - $38,400 = $3200
Basically, by paying every two weeks (26 payments) vice twice a month (24 payments), you make two extra payments per year at $1600 per payment, so you overpay by $3200.