Newer CPU are more advanced than older ones, especially processors made for notebooks. Usually, when you are on battery mode, CPU will automatically lower it's speed, in order to save battery life. But, when you plug it in, or when you play a game, CPU will get a boost in order to run the aplications faster.
You asked about intel hd 4000 and it's frequency, well according to intel (http://ark.intel.com/products/72057/Intel-Core-i3-3227U...) , the GPU can max out at 1.1 GHz. It will normally work at 350 MHz, which is enough to run Windows 7 for example.
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