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Not being condescending here, but google it:

http://www.google.co.uk/search?q=bufferbloat

Some very good descriptions and resources there. VM uses excessively high (bloated) buffers which causes excessively high latency and jitter (your graph) when your connection is under load. I can't explain it better than Wikipedia already does, so quoting el wiki:

Bufferbloat is a phenomenon in a packet-switched computer network whereby excess buffering of packets inside the network causes high latency and jitter, as well as reducing the overall network throughput.


This problem is caused mainly by router and switch manufacturers making incorrect assumptions about whether to buffer packets or drop them. As a general rule, packets should not be buffered for more than a few milliseconds. Any more than this can lead to TCP's congestion-avoidance algorithms breaking, causing problems such as high and variable latency, and choking network bottlenecks for all other flows


When bufferbloat is present and the network is under load, the symptom is that normal web page loads can take many seconds to complete. Any type of service which requires consistent throughput (whether low or high bandwidth), be it VoIP, networked gaming, text and video chat programs, and interactive application such as remote login become next to impossible.


While latency has been identified as more important than bandwidth for many years,[3], the falling price of RAM encourages the use of larger buffers and exacerbates the problem.

"Normal" connections with "normal" buffers don't do this. The excessive buffering VM uses degrades your service and everyone else's, while being a pain in the proverbial basckside for network administrators. Whereas heavy downloading like you mentioned should only increase latency by 20-40ms, on VM it increases by well over 100ms and anywhere up to 1500ms in bad cases.