Recent research suggests that BP readings aren't as accurate as they should be. As a result, many people are easily misdiagnosed. Usually mistakes skew to the high side, so people are sometimes put on medication unnecessarily, or given doses that are too high.
Our BP fluctuates throughout the day. Evaluating it is like shooting at a moving target. All sorts of things can cause BP to rise or dip. Did you know, for example, that you shouldn't smoke, exercise, or drink caffeine for the 30 minutes before being tested?
The possible variations don't matter too much if you're clearly hypertensive, or clearly normal. But most of us are in the middle "gray area."
A standard way to measure BP has been recommended by the American Heart Association. Here's a summary: