Blood pressure is a wierd thing. I had mine checked for the first time in a very long time a couple of years ago. I was quite shocked when it came back as 170/120!! I'm not overweight, cook all my own food sans salt, don't drink excessively, exercise a lot...
Did the 24-hour thing c/o the hospital which measures it every half hour for a day. Most of the time it was perfectly normal. GP even said that taking BP on the spot is practically a worthless exercise as it can go up and down at the drop of a hat, hence averaged readings over a period of time were the only meaningful way to measure.
Went for my initial medical and it was 150/100. Told the AME about the 24hr thing and she suggest I just read a magazine and chill out for 10 minutes whilst she did the paperwork. Took it again - perfectly normal.
I'm sure everyone's different, but it does seem like one of those things which can have a mind of its own!