Same cpusets assigned to new instances even with compute having other free cpusets

Issue

• I have created a VM using nova and vcpu placement is proper in nova configration file generated, But once the VM is created, libvirt is assigning its own cpuset to the VM and not the expected one .

Different cpusets are defined in nova libvirt xml file.

[root@compute-2 instances]# grep cpuset 1b2aa972-9dce-4732-b4b0-2f622bb20f09/libvirt.xml
  <vcpu cpuset="12-23,36-47">7</vcpu>
[root@compute-2 instances]# grep cpuset 2d62e0fe-cf0d-4f81-ad29-2722dc3e0ead/libvirt.xml
  <vcpu cpuset="0-11,24-35">16</vcpu>

But the assigned cpuset is different when checked while running:

[root@compute-2 instances]# virsh dumpxml 12 |grep placement
  <vcpu placement='static' cpuset='0-11,24-35'>7</vcpu>
[root@compute-2 instances]# virsh dumpxml 13 |grep placement
  <vcpu placement='static' cpuset='0-11,24-35'>16</vcpu>

1) Why are the VMs getting stacked on a single cpuset <vcpu placement='static' cpuset='0-11,24-35'> when the other node(s) are free?
2) Why the cpu allocation is not changed once allocated in the lifetime of the VM according to the load.