Vitality rewards not making sense!!

To whom it may concern, I have a query regarding the vitality rewards. I am sitting with X amount of points which I can redeem with no problem. Now I am sitting with an additional diamond but cannot do anything with it? I called the Vitality help desk and they informed me that I cannot redeem anything with 1 diamond and need to wait until I manage to get a second one. The diamond expires in March so if I do not get a second one then that expires... This does not make sense to me. Why can I not use the diamond and add it to my points for a purchase? What is the purpose of getting a diamond and not being able to use it to my convenience? One should have the choice of keeping it hoping to get an second or third OR exchange it for a point value. Can someone please explain this to me?

Thank you Regards