IIS changes certificate

I have to sites:

http://my.site.com  (only redirect to www.my.site.com)

https://my.site.com  (only redirect to www.my.site.com)

and

http://www.my.site.com

https://www.my.site.com

I have for both sites a certificate. (my.site.com /www.my.site.com)

I set for the www site the www certificate and after that i set the non www certificate for the non www site,

IIS is changing the certificate of the www site to the non www certificate.

Why is this happening and how i can solve this problem.