Hi, Tupinambis.
Like Dr Adequate said, plant development allows a lot more deviance from the "norm" than animal development. It isn't all that unusual in plant evolution to see a feature or even an organ switch locations on the plant between closely-related species.
But, even without that, I could see a bowl-shaped flytrap easily transitioning over evolutionary time into a pitcher by simply becoming deeper and abandoning the sticky stuff. I could also see curled leaves easily transitioning over evolutionary time into a pitcher.
-Bluejay (a.k.a. Mantis, Thylacosmilus)
Darwin loves you.