Comment 3 for bug 1974078

Here you can see the method
consume_from_volume, which updates the attributes for the pool (PoolState) object.
This updates the allocated_capacity_gb, free_capacity_gb.

https://github.com/openstack/cinder/blob/master/cinder/scheduler/host_manager.py#L336-L353

The pool's capabilities is NOT updated, and hence the get_pools call returns out of date values
for the pool between get_volume_stats() calls.