Is my employer over-charging me for imputed income?

Hello,

I recently added my non-dependent partner as my domestic partner on my health insurace.  My benefits representative said I'll have a deduction of  $273.60 per pay period which is my imputed income.  As I understand it, imputed income is the value of the benefits to my DP that I'm responsible for paying the taxes on. So if my imputed income was $273 per pay period, wouldn't I only owe the tax on that?  I looked up how much my employer is contributing to my insurance premium for my DP and it's about $250.  So how am I being deducted far more than the entire employer contribution?  Am I missing something? It seems like something must be incorrect here. Any help much appreciated!