To some extent, this is normal and necessary for the body to adapt to different situations.
Systolic and diastolic arterial blood pressures are not static but undergo natural variations from one heartbeat to another, and throughout the day (in a circadian rhythm). They also change in response to stress, nutritional factors, drugs, disease, exercise, and momentarily from standing up. For a meaningful comparison, try to measure under similar conditions. For example, take daily measurements at approximately the same time, on the same arm, or as directed by your doctor. Always rest for 5 minutes before you measure your blood pressure.
If you are in doubt, or if your blood pressure is consistently to high or too low, please consult your doctor.