Cool findings by Studio Neat, makers of Frameographer:

What Apple means by “dynamically selected intervals” is they are doubling the speed of the time-lapse and taking half as many pictures per second as the recording duration doubles. Sounds complex, but it’s actually very simple.

Make sure to check out the table with numbers and the videos. In typical Apple fashion, the default solution is clever and simple, leaving room for third-party apps to offer more.