The fluctuations come from the algorithm itself. If you the algorithm is mostly fast components then your hash rate will be much higher than when you have a bunch of slow components to the algorithm.
Yes it changes every minute but you will have some periods of time where it is real high, real low, or something inbetween.
Maybe that's what makes the RVN algorithm strange, and it's always been the case, although it's been upgraded many times for mining tools, and if the value of RVN is enough to attract investment from ASIC, it may become more stable in the future.