Yes, there are a whole host of things that can affect HR during one of those workouts.
As you say, changing position is definitely one. Eating / drinking is another.
My biggest spike so far, someone giving me a fright by unexpectedly coming into the shed!
Towards the end of my base training, I got to the stage where the chart usually looked like this
After a bit of build work, that has increased my TR AIFTP, the charts now look like this.
The ceiling has been raised and I have a considerable amount of work to do to fill the gap again.