Resource Percent Availability at task resource assignment

Hello,

Scenario:

While doing the resource assignment to the project a percent allocation say 50% allocation is done for the resource. 

When I go to the project and open the resource plan its is correctly showing me duration as Partial, CM % as 50%. The weekly allocation too shows CP as  40.0 hrs and CM as 20.0 hrs. But when i try to assign resource to a particular task in WBS the Percent Availability Plan & Percent Availability Actual both show 100.

I want to know how the Percent Availability Plan & Actual are calculated, ideally percent availability plan for the project/task should have taken the CM% for that resource for availability % calculation.