It is to do with the processors ability to boost clock speeds when the conditions are right to do so. Mainly that the processor is not getting too hot. When gaming, your processor will hardly every run at 100%, so the processor does not get as hot. As rendering uses 100% processing power, the cpu will get hot causing the clock speeds to fall down to a level where the cpu can maintain an acceptable heat as to not cause potential damage to itself.
I assume you are using a stock cooler? If you want to see higher boost clock speeds then you would likely need to invest in a more expensive air cooler.
