If you are pushing the CPU it ramps up to 1200 to process the information at a desirable speed. Simple tasks and uni-tasking can often be done at the 350 slot which is why it is so high. Also I believe when you wake a sleeping phone it automatically goes to Max to perform the needed tasks of waking then settles back down.
You are scrolling through a homepage and it stays at 350MHz - as you do, you launch an app, your kernal immediately shoots to MHz to launch the app quickly and once it is opened it goes down to 350MHz. During a game, even, it will only be at 1200MHz for a few short seconds.
So it is completely normal to have mostly 350MHz and 1.2GHz.