It's because android does not take advantage of the GPU to render the scrolling effect. The iPhone does. This was a design decision early on, and has caused plenty of ire.

http://code.google.com/p/android/issues/detail?id=6914

Another reason this was a poor decision is that since the phone has to use the main CPU to render the scrolling effect, the battery drains very quickly while scrolling through a webpage. Scrolling around on webpages for around 20 minutes usually drains my battery 25% or more on my HTC EVO.

Rumor is that android will use the GPU for its UI in honeycomb, and looking at the tablet they were showing off, the effects and scrolling looked very fluid.