Another fun factor I learned about five years ago is temperature. My gym was offering a complimentary fitness evaluation, which among other things included a blood pressure test. The trainer was horrified to see 140-something over something equally terrible, and started explaining how she'd have to refer me to a doctor and advise that I refrain from strenuous physical activity, until I showed her a report from my annual physical a few days prior with 106/70. Turns out that walking to the gym in shorts and a T-shirt in late January causes enough vasoconstriction to really screw with some measurements.