Hi and welcome the voltage is showing lower in CPU-Z because of something called V-Droop there might be some options in your bios to help with it but if you are not having stability issues I would leave it be and call it a day. Run Prime95 for at least 6 to 12 hours to check stability? Also I find that OCCT will find instabilities quicker.
Read this page to understand V-droop and also some great info in this whole article. http://www.anandtech.com/show/2404/5