Why does my phone say I have less space than iTunes does? According to the usage and about settings on my iPhone, I have 2.2 GB of space left. However, when plugged into iTunes, iTunes tells me the phone has 4.62. GB free. Is there a reason for this, and which one is the correct amount?

(16GB White iPhone 6, iOS 8.4)
You could have "phantom" photos and/or videos on your iPhone.  Removing them manually worked for me.  Look at camera roll "usage" number does not match photos in the app for a manual solution.