A row with your other half, an extra pre-ride espresso, nerves or being late for the Ride Captain’s briefing can all raise your heart rate. Don’t forget it can also be impacted by illness, hydration level, air temperature and altitude. If you’re especially fatigued from a hard training block, this can also flatten your heart rate.
For longer rides, there’s also a physiological phenomenon known as cardiac drift where, over the course of a long ride, your heart rate will rise even though you’re not working harder. This effect can be as pronounced as 15% and, although it can be managed with optimal hydration, it can’t be completely negated.
When riding indoors, many riders, in relation to their perceived effort, find that their heart rate is typically 10-15 beats per minute lower than it would be outside. There are a number of reasons for this, such as reduced recruitment of core/stabiliser muscles and simple boredom but, as long as your pain cave is well ventilated, you have a decent fan and your position is the same, your power numbers should be similar to riding outdoors.