Short answer: Because you are paying ahead for the months of July and August. Your monthly premiums are calculated on an annual basis, and divided by the number of remaining payrolls.
If you are a returning employee (or begin employment in August), 12 months of premiums (September through August) are divided by 19 payrolls (September 15 through June 15).
So what does this mean in real numbers?
Let's say you are an employee that is paid during the school year (19 pays per year) with a benefit of single coverage. You elect a two-party health plan. The monthly premium is $1368.46.
$1368.46 times 12 months = $16,421.52 annual cost.
The district pays the single premium portion (657.73x12=$7892.76). Your portion is the balance (16,421.52 – 7892.76 = $8,528.76).
$8528.76 divided by 19 payrolls = $448.88 deduction per paycheck.
This way the premiums for July and August are already taken care of and paid for so you do not need to worry about coverage during the summer months while you are off.