Why would a different accelerometer surprise you? The entire SiP is different between the Series 3 and Series 4. One source say the accelerometer in S4 has higher dynamic range. [0]

[0] https://www.anandtech.com/show/13364/apple-announces-the-app...

They specifically said in the announcement that it was due to new tech.

Consider that to detect falls reliably, you might need an accelerometer with a higher range than that was in the original watch.

Might also have needed a higher sample rate.

For battery life concerns, I can see them implementing some of the fall detection logic in hardware as well. For example, a high shock or shaking could trigger the fall-detection software to wake. Although, with the 60-second period, they probably wake the main processor often enough anyway that hardware detection might not save much power.

> which seems questionable

Maybe because they added new hardware to enable it...?

Series 4 is very different hardware. I'm not surprised.