There are many things that could be causing the fluctuations in your blood pressure.
Exercising-your pressure may be up when you first entered the store because you walked in from the parking lot and then it is lower later because of your leisurely stroll through the store.
Other factors that affect BP:
Eating
Caffeine
Smoking
Overweight
Medications
Hereditary history
Ethnic makeup
Outside temperature
If you are seriously trying to lower your BP which based on those numbers you should, then check out my post
No more excuses!