I’m not sure if you are worried about the bouncing CPU usage or the 100% usage. If one or more of your cores are at 100% when encoding then you are encoding at the most efficient level for the CPU speed you have. Whether it is one core or more at 100% depends on whether the encoder can use multiple threads.
If the encoder used doesn’t make good use of sub-threads then you will see erratic use of one or more cores (total usage bounces around). If the CPU(s) are choked for memory or are faster then the IO (bus, memory speed, CPU cache, HDD read/write, etc) then all threads will bounce as they are starved for information to process.
It looks like DVDFab Mobile uses more than one encoder under the covers so different profiles could have different CPU usage on the same machine and may handle different types of CPU(s) better or worse.