repeat measurement difference

You need to think of an HRV measurement more like a blood pressure than an HR measurement (in fact HRV and blood pressure are regulated by exactly the same parts of the nervous system).

Many people have heard of 'white coat hypertension' - that's when your blood pressure is higher in the Doctor's office simply because you are anxious.  That same anxiety or lack of relaxation will also lower your HRV.  

We strongly recommend doing one reading and not repeating without leaving a 5-10min break if you really think the first measurement was wrong.

The breathing rate is also very important for consistency, as explained in the following quote from Dr Liz Miller, a former neurosurgeon, psychologist & author who is also an ithlete user:

The emphasis is getting people to breathe right, which maximises HRV because breathing profoundly affects HRV -
Heart Rate Variation is largely due to the changes that occur during breathing - breathe in - Heart Rate goes up - breathe out - Heart Rate goes down. These changes are healthy, normal and partly relate to the mechanics and partly to the autonomic (sympathetic and parasympathetic) nervous system. The greater the changes between breathing in and breathing out, the healthier and fitter your heart and lungs and autonomic system.
In other words heart rate variability measures the amount your heart rate changes from moment to moment. Most of this difference is due to breathing patterns.
Any change in mental state, even if you are an Indian meditation guru will change your pattern of breathing - the two are inextricably linked! Anxiety produces shallow fast breathing, calm produces slow deeper breathing.
The slower and more consistently you breathe, the greater your HRV at any moment, up to a maximum that relates to your own personal fitness and gives you an ithlete number
Inconsistent breathing, even slight, profoundly affects HRV. Although most people learn subliminally that to get their best reading, they need to follow the animation on the screen, and breathe in a slow and measured fashion, it is possible to mess with your HRV by fractionally altering your breathing. This will happen if you are anxious, tired or otherwise stressed.
From a user point of view - HRV needs to be a peaceful ritual. You sit and relax for a couple of minutes, watch the graphic on the screen and then when you are feeling calm, join in breathing with it and then when you are happy your breathing is nicely synchronised, switch on the device and measure your HRV