Why is the chef only cooking amount of food i.e. there 14 people but only cooking for 10-12 people...

They can only make as many meals as food allows. Ie 40 per meal, they have 320 food available- they can only make 8 meals. Even if you have 14 people. That's why you have to manage the hunter cabin ability to provide food and number of survivors so carefully.

The kitchen will have a maximum amount of meals that can be made and stored before eating. Upgrade the kitchen building to increase this cap.