Why might home blood pressure monitor be giving incorrect reading?

[Corialanusburt]

DP has been asked to do daily blood pressure readings at home with a monitor. The readings he got on his first go are higher and more variable than those he's had when the nurse has measured it.
does anyone have any idea why this might be? Could he be wearing it in incorrect position?

Is it an upper arm one and is it the right size cuff?

you need to follow the instructions well. always use the same position of upper body and arm.

You need to check the cuff size is correct, it should say in the packaging what arm size it is suitable for. A cuff that is too small can give a reading that is incorrectly high. You can buy larger cuffs separately if necessary. Also check the positioning, there should be a dot or arrow indicating where to place the cuff over the artery. Rest his arm by his side on a pillow while taking the measurement. Check the batteries also. Failing all that, it's possible the machine is calibrated incorrectly.

Another one coming to say cuff size. Really makes a massive difference weirdly!

Another option is the machine might be faulty - they do really need calibrating after a few years I think? But if this is a new machine then that's obviously not the problem.