Alternative component

Hi,

I have maintained the alternative component as per the following

Component A

Priority                 : 1

Strategy               : 2

Usage probability : 50%

Component B

Priority                    : 2

Strategy                  : 2

Usage Probability  : 50 %

Stock Available for Component A is 1000 kg and B is 0 kg

Now when is run the MRP and convert planned order to production order for header materials then in production order system distribute the stock in following manner

Component A  - 500 kg

Component B - 500 kg

My understanding is system should display as per the following

Component A : 1000 kg

Component B : 0 kg

Please comment why system is reacting like this or I have missed something.

Regards,

Sun