More short bursts of power? On SOLO rides my HR goes a little higher outdoors, from short power bursts due to traffic or corners or more frequent standing. But usually my HR is only a little higher during SOLO workouts, because my power graph looks very similar inside vs outside.
Group rides? Yeah generally I see a lot more bursty power on a group ride. Higher highs and lower lows. That will drive my HR a lot higher.
For example:
you can see I was riding by myself on target (a range of power) until around mile 11. Then I saw the group ahead and increase power to slowly catch them around mile 18. So the first half of the power graph looks pretty similar to riding inside on the trainer. And HR is similar inside vs outside.
Once I’m with the group, my power graph gets “messy” and HR tends to go up. Around mile 30 I split off to return home and you can see it starts looking more like inside again.