It has nothing to do with market fluctuations. I've been using CamelCamelCamel for years now, and you can see wild fluctuations on an hourly basis, sometimes by more than 50%.
Amazon and its sellers do A/B testing of price points all day long, and if you're just a casual shopper that happens to want the product at that time specific time, you may end up being unlucky, and paying a few bucks (or sometimes dozens) more.
You're suggesting that the worldwide price of tea or market conditions are causing these nearly monthly fluctuations, between two set prices? I'm skeptical, the graph doesn't suggest that to my untrained eye.