Task Manager shows how busy the CPU is. This is unrelated to the "maximum" performance of the CPU. So if you are using 80% of the CPU at a normal performance level, and the CPU clocks back to 50% performance (possibly due to temperature, in your case), the Task Manager should show 100%. The processes want 80% of full performance, but they only have half of the full performance, so they will use 100% of what is available. Task Manager will show 100% because the CPU is 100% busy (but at a lower performance level).

So in your example, if CPU throttles back to 20% performance when usage is 30%, Task Manager will show 100%.

Can CPU temperature cause the CPU to appear to spike to 100% in Task Manager?
I have a Samsung Galaxy S2, running Android 4.0.3. 

I want  to install Nova 3 Near Orbit on this device. I need 2 gigabyte of memory to install the game, but I only have 1.97 gigabytes in total.

How can i get the other 3 megabyte? I have 11 gigabyte of space on my phone, and I also have an SD card with 14 gigabyte. 
