Hi Karatant, I was always advised....as I suffer from high blood pressure...that it is the lower figure that is more important, as that is the rate at which blood passes through your heart when at rest. The upper figure being the rate it passes through when on a beat. Anywhere between 60-80 for the lower figure is acceptable, yours being 64, so OK maybe slightly low but still within normal range. Hope this helps to explain what it all means.
It is in the lower range of normal though mine is often lower (down to 90/60 something sometimes) but it's still within the normal range and, as said above, much better than being high :)
More of a problem if there is a marked change or sudden drop so a number of readings over a period of time can be more reflective of your blood pressure in general.