It would be good to get some clarification on this, I agree, churn gave me the same impression of change as well.
If you hit "More..." above the graphs you'll get more information about the tab. My my understanding is that its showing what percentage of cache hits are coming from which volumes. So you should be able to use this in conjunction with the cache utilization to hone in on volumes that are utilizing cache that may not require the latency benefit of reads being served from SSD. I've only really used this tab when looking at an array that's near 100% cache utilization.