timestamp frequency greater than than 1 Hz

Hi

Apologies if this is covered elsewhere in the docs or the forum - I have looked, but no luck.

General googling repeatedly reports that Garmins timestamp data once per second (ie 1 Hz) into the FIT file however a sample file I have from a ForeRunner45S has timestamps less regularly than this
e.g. see screenshot..

... so sometime consecutive seconds but sometmies longer intervals (upto 5 .
I'm trying to understand why this is. Is it simply that if none of the data points change from the previous timestamp that it's not repeated?
I struggle to believe it's as simple as this because whilst heart_rate might not change that quickly, I'm pretty sure that other parameters such as distance will have changed in, for example the intervale betweemm 10:46:21 and 10:46:26.

I'm looking to use this data with academic research (especially interested in heart_rate) so would like to have a clear understanding of what's happening here.

A related question, do all watches log data at the same rate or are some better at logging closer to every second?

Many thanks,

John