Why wouldn't he contribute the 2017 max of $18,000?
It’s not his contribution. It’s the company match.
The $16k figure sounds like the amount that Apple contributed on Cook's behalf via matching funds.
In 2017 companies may only use up to $270,000 when calculating company matches to a 401k * whatever their max limit is for normal staff. 275 * 6% = 16.2k which sounds about right.