it got better
The newer ipod touch has a better screen so everything shows up better. Plus they have improved the battery life.
That's like asking "why did apple make a different ipod". Technologies tend to improve with time. But... Seriously?
how the anemometer changed over time
The laws of physics have not changed over time. Our understanding of them has changed over time.
how has coal mining changed over time
People changed over time by growing up.
no they have changed over time. no they have changed over time.
how has neils bohr model changed over time
well it has changed over time bye people dancing it different styles.
Over the last ten years the iPod has basically changed in: Memory, performance, and user interface. Some of the fist iPods only had 5 GB memory. Now they have iPods as large as 160GB. Also the fist iPod used a click-wheel and was large and heavy. Now there is a Nano that is small and holds a lot and the iPod touch that uses a 3.5 Multi-touch display. To see the changes try Wikipedia or Google image search: iPod, original iPod, iPod touch, iPod classic to see the physical changes.
everything has changed on the ipod
how the seismograph change over time