| It's a bit psychological hardwiring that in prehistoric times would have helped you survive. The brain is designed to make you more hungry according to the amount of available flavours. This is so that overall you eat a wide variety of things and thus get a wide variety of nutrients.
That's why you feel fuller when you eat bland food and why you eat far too much when you have a BBQ or a buffet. Or why after a big meal, you've always got room for dessert. It's not that you're genuinely hungry, it's just that your brain knows you've got something that tastes different to your main meal lying around.
Also, the waiting time means that your brain has longer to react to a full stomach. It takes a while for your brain to catch up with the contents of your stomach, which is why for the first 20mins of eating, you're starving, and then suddenly you feel less full.
That's why if you ever do an eating challenge, NEVER pace yourself. Just ram the whole pizza/burger/steak in there as quick as you can and then go hide in the toilets for a bit after collecting your prize. It's also why people in the 50s were told to chew their food thoroughly - you eat less but feel just as full.  | 
