You have bad information. Citrix provides a equation to answer your underlying question about CPU assignments.
XenServer 5.0 Update 3 Admin Guide (p. 140). http://support.citrix.com/article/CTX120713 :
Provision just enough Virtual CPUs (VCPUs) for each VM's workload.
Don't give a VM more VCPUs than it needs to do its work. If the server in a VM only needs around 300 MHz, it only needs 1 VCPU. Only give it one. If it regularly has 3000MHz worth of work on your 2.5GHz quad-core box, give it two VCPUs; but don't give it three or four.
The number of VCPUs a VM has should reflect the amount of work it has to do. If your VM has 2 VCPUs, it should have more work than 1 physical core can do in a timely manner. If it has 4 VCPUs, it should have more work than 3 physical cores can do in a timely manner.
As a formal application of these principles for workloads that require multi-VCPU virtual machines,
allocate VCPUs to maintain this constraint: (V - N) <= (P - 1), where:
V = Total VCPUs across all VMs
N = Number of running VMs
P = Number of Physical CPU cores