The amount of charge the battery gets is the amount the device can draw from the charger (that's fixed inside the device, no matter how much more current the charger is capable of supplying) minus the amount of current being used by the device. If you're playing a heavy game, or are on a web page with a lot of Javascript, the tab is going to be using a lot of current, and there won't be much left to charge the battery.
That's not just normal, it's electricity 101 - Kirchoff's Law (all the current entering a junction is equal to all the current leaving the junction) on a larger scale.
If the tablet can draw 1,200mA, and is using 1,000mA, the battery is getting only 200mA charge, a far cry from the 1,200mA it would get if the tablet were off. (And it will take 6 times as long to charge.)