My mum had a similar problem - was told to eat less salt (she doesn't put any in her food and rarely eats processed food), lose weight etc etc. Turns out she just got incredibly nervous at the doctor's. So they've got some sort of system now where she measures it at home or something.
I had an evil nurse once who got me totally panicked and told me I couldn't have my implant in because my blood pressure was too high. I then went in to see the doctor, she took it and it was fine. If you breathe and concentrate on the number on the machine, I find it always settles. If they're doing it the old-fashioned/proper way just .. breathe! SOrry!
I usually get stressed cause I hate that thing that expands on your arm.