Does mobile charger and buck converter have any basic difference?
Edit: well, apparently this isn't the answer, according to the asker. The charger has no communications electronics in it either. Leaving it here because it is an answer to what makes chargers different, just not the right answer in this case.

In all likelihood, your smartphone charger contains active electronics designed to communicate with the smartphone and negotiate how much current can be drawn. The USB specification requires that host devices (such as your charger) output no more than 100mA unless the device on the other end requests more. Since your battery is just a dumb battery, it doesn't have the electronics to request more current, and your charger provides its default of 200mA. (few chargers actually adhere to the 100mA limit. Most go straight to 500mA, the maximum allowed in standard USB 2.0. It's odd that yours goes to 200mA, but not unbelievable.)