The graph is doing average for the last 5 seconds
You are presuming there's a perfect mapping between 1x vCPU and 1x physical CPU. Xen could schedule one vCPU into various physical CPUs. Likely, one of your vCPU load is spread on more physical CPUs.
Remember: for Xen, your whole VM is like a process. It will be scheduled on "real" CPUs in a way that might be not "what you see" from the guest.
