tech checks eye pressure and its fine, then a few minutes later doctor checks again and its high

I have pre-glaucoma (my field of vision is normal, but the nerves in the back are damaged).  I've been to the doctor several times.  Each time the tech checks my pressure and it's fine (13-15), then a few minutes later (5-30) the doctor comes in and checks again.  It has always been high (18-20) when he does it.  He says she's not doing in right and she says he's not doing it right.  Is is possible that he makes me nervous enough for my pressure to go up that much in such a short amount of time?