Yes it's normal, I just learned this too myself to much surprise, you'll see your core cocks are boosting up and down as it happens, just set your fans response time to be less aggressive and your fan curves to ignore the spikes at lower temps and ignore it, it's completely normal and as said above, as long as you're staying within reasonable temps under load then all is good. It's a feature of the chips that allows them to be more responsive by not completely idling cores so there's less response time when a load occurs.
I'm sure someone else could give a more technical detailed description if they wanted to.
This is a useful watch...
