Usage means how much time the screen has been on and been using the phone. Standby time means since the last charge. So if you fully charge the phone and unplug it, your standby time will start to tick. Even when your phone is in sleep mode, it will continue to tick. For me standby time doesnt really matter, its the usage that really matters
In this case your standby time was 8 hours, that means 8 hours ago you unplugged your phone. And 6 hours you used your phone. Both does not add upto 14 hours of usage.