Why is iphone 6 ios 8.3 2.6gb available in mobile and 5.46 gb available in pc?

When I checked it in my settings in the phone settings>general>usage 2.6gb is free. When I connected it in PC and synced it, 5.46gb is free. Why is that happening and how can i make 5.6gb free appear on my phone?

Make sure that when you are in iTunes on your PC that there's no changes being made/synced (such as removing photos or music from the device) because that will adjust storage size, also look for big files on your iPhone such as Music, Photos or Videos as they will be the ones that don't sync with iTunes and make it look like more space is on your phone