Why does charging leads to battery losing charge at times I have a LG Optimus S Android on Gingerbread (The one on the cheaper side with least specs for an Android).

I have found that when I use the USB charger connected to my computer, at times instead of charging my phone, the battery gets drained. (Also I perceive that the battery gets drained faster than normal phone usage at such times)

While I have a fix which is to restart my Android phone. Could someone explain why such thing happens?
Now your typical wall charger on a smart phone provides 1A of charging current. The USB 2.0 on a computer will only supply a max of 500mA, that is half as much. So if your screen is on and you are using the phone then it is drawing more power than it is taking in since the screen is the largest drain on the battery, therefore the battery will drain while charging on a USB connected to the computer.