Value of Ethereum vs Bitcoin

I recently started doing some research into cryptocurrency and while doing some growth comparisons between different currencies I decided to do a comparison of Ethereum/USD to Ethereum/Bitcoin (ETHBTC). I noticed that while Ethereum's dollar value is rising substantially, its value in comparison to Bitcoin has been declining almost inversely to ETHUSD since October. I was just wondering what this speculation could mean, as what I assumed is that this means Ethereum's overall value is unstable and is simply being inflated because of the current graphical trends.

I have much more learning to do and it would help a lot if someone with more experience could provide insight into this.