Activity Monitor does not distinguish between cache hits and misses, and counts the number of reads and writes to the host filesystem rather than summing the requests to the physical devices. Parallels presents the guest with a virtual filesystem, and the device driver for this filesystem does no caching of its own, as this would waste memory due to double caching. Instead, it merely passes requests through to the real filesystem on the host. These requests (read or write) produce system calls, and this is what is being reported in Activity Monitor regardless of whether they use the page cache or physical SSD.

One way to test this would be to use DTrace, which can distinguish between requests to the filesystem and activity that reaches the device. The scripts "hfsslower.d" and "iosnoop" are here: http://dtrace.org/blogs/brendan/2011/10/10/top-10-dtrace-scr...