I have noticed that on other usb charging besides the Dart. I don't think pluging into a usb port gives the same amount of charge as the actual phone charger that has the usb port in it. Maybe I am wrong but I have noticed the same thing as you. Even my desktop computor usb doesn't charge as fast.
On my desktop I have 1.0 on the front and 2.0 USB on the back, the 2.0 charges way faster then the front. I noticed that if I use a Cigarette charger(12V) it charges much faster. Anyone else noticed this?