It can change if you:
Take a measurement after you’ve been walking or moving. Let’s say you take 3 measurements in a row. Each of them takes around 2 minutes. Add another minute to check the results. That would be a 10-minute difference between the first and the last measurement if you use camera, or a 15-minute difference if you use a heart rate monitor. When you change the position, you body starts to adapt to new conditions which affects the measurement results. That is why if you’ve been walking or changed position you should wait for a couple of minutes before taking a measurement. Your body will adapt and the measurement will not show different results.
Take back-to-back measurements right after you wake up. The reason is the same: during this time, your body has time to wake up and trigger shifts in your sympathetic/parasympathetic balance.
Remain in the same position for too long. Having to maintain the same position leads to higher stress for the body and will affect results.
Check Facebook, email, or had a phone call, etc. during or between the measurements. A strong emotional response could affect your mood, emotional state and results.
Are in a borderline state. It means changes are happening in your body during the measurement and it affects the result. For example, if you come home after a hard day you body starts an intense recovery process and it is reflected in the results.