Commonly used arm positions can overestimate blood pressure readings: study

One of my attendings has a running joke where he takes fresh interns in July and tells them "I will take you to <most expensive restaurant in my city> if you can take this patient's blood pressure correctly." He's been doing this for 30 years and no one has been treated to dinner yet. He's looking for the standard rest time, arm at heart level, everything.

After reading both the article and your comment I have to say I'm somewhat confused. I don't think I've ever had a medical professional not take all the right steps when taking a reading, is it common in some context?