Heart rate variation is a real thing. Wiki it.
I like Azumios Instant Heart Rate. Have used it for one year plus it works well and gives verifiable results. This app seems to give reasonable stress level results but logs them with errors. I ran around and took my finger off the lens to produce high-level stress results. As expected it gave high-level stress readings, but logged them as 1% readings with my previous comments. 2 stars.
IndyWindy about Stress Check by Azumio - Lite