Barttery Charing Differece

Sometimes I charge my battery at work via USB cable. I don't have iTunes installed on my machine at work. I've noticed that my phone will get ever slightly warm to touch even though it hasn't charged all the way. When I charge my battery at home on a Mac with iTunes (this is the machine I sync to), the iPhone never gets warm even when I leave it overnight for a charge. Has anyone else noticed something like this?