Forerunner continuously updates it position every second, but data is saved in trackpoints much less frequently. Here's a summary of a post on GarminF that describes how the process works on the 201/301 models.
Unlike some Garmin receiver models, Forerunner 201/301 cannot be set to store trackpoints at fixed intervals. They use an "automatic" mode, which only saves a trackpoint when there's a significant deviation in the course you're traveling.
The algorithm works something like this:
This is why more trackpoints are saved along a twisting route than one that is relatively straight. Fine tuning can further improve track log accuracy by reducing tolerance levels when a curve is detected.