UIWebView leaks, JS Garbage Collector & WebCore VMs
I'm unsure if I would immediately consider this a problem. Those VMs only start when you add a web view to your view hierarchy and load a request with it.
It may just be the OS that keeps those resources alive in case they will be used again.
I tried to replicate your code that cleans up the data and had drops of 50%.
The JS Garbage collector was always at around 3MB minimum, but had maximums of 8MBThe WebCore purgeable data was always at around 700KB
You may also consider that this may be related to bad JavaScript that continues to live because of leaks in the JavaScript code.