Taking a BP

I went to the doctor two weeks ago and when she takes my bp I pull up my sleeve and she says to me "you don't do that,that could alter your reading". She also said"You should know that,you are a nurse right?" She might have been a recent immigrant,so I just figured that maybe she didn't understand she just said an ignorant remark about my skills.Since WHEN did this change about the bp? I,my co workers,bosses all have rolled up patients sleeves and then out the bp cuff on. As a matter of fact,that was my first time seeing it done that way.
It's my understanding that taking a blood pressure OVER the sleeve can alter the reading.

I have no idea, statistics, studies, etc. I just figure taking a blood pressure over a thin sleeve cannot alter a blood pressure reading? How could it? However rolling up a sleeve could slightly act as a kind of tourniquet and that (in just my gut level thinking) could alter the reading. However I can't imagine, unless you carry out either to some extreme, (a really thick sleeve or a really tight tourniquet effect) would make a big difference?