Unless you have many runs’ worth of data and it is consistently lower I would say that 7bpm difference is negligible. Heart rate varies wildly day to day, and any number of small things can cause a 7bpm swing. My guess is your brain is more stimulated outside.
If it really is significantly lower over many data sets and you’re training by heart rate, bump your treadmill speed a bit I guess. I wouldn’t over-think it though. RPE is your best tool to compare inside and outside run speed.