From my personal experience in working with eCommerce for several years, fluctuations are quite common especially on the results lower down. It can also depend on what kind of industry and content you deal with, some content might fluctuate more than others. I suggest looking at your rankings every 4-6 weeks rather than every few days, looking every day can be deceiving. Also, try tracking your competitors’ ranking and see if they fluctuate too because if they are fluctuating too it’s definitely not you, it’s google.