Blood pressure and pulse rate depend on many factors that may also have short-term influence:
◾ Physical influences
◾ Mental influences
◾ Physical movement
◾ Muscle tension
◾ Noise etc.
The type of measurement is also relevant:
◾ The cuff being too tight/too loose
◾ The cuff not being at heart level
◾ The intervals between measurements being too short
◾ A lack of visual or auditory patient isolation
Device tolerances also play a part of course. With devices that have a manometer (pressure gauge), the parallax error can also occur.