Same here. It happens for a lot of curly heads. For me, my hair is very fine so I think its mainly because the weigth of the hair pulls down the curl at the top and it appears straighter.

There are ways to prevent the flat top and create more volume. You can try plopping, the criss-cross thing mentioned above or any other method really to lift your hair away from the scalp and let it air dry without the weigth of the rest of your mane pulling on it. Layers can also help a great deal to avoid the "pyramid head" look haha.