Mrs. Owens comes to your office for a visit, Her blood

Mrs. Owens comes to your office for a visit. Her blood pressure is usually in the normal range. However, last week while she was waiting for a prescription at the local pharmacy, she tested her blood pressure on a machine that was recently installed there and found it to be 145/100 mm Hg. Today, her reading is 120/80 mm Hg. What would account for the different reading?