$15.00 Discount for Access Credit?

I'm on the mobile share value program with my sister and we're both billed $40. for the plan. However, I receive a $15.00 credit "Discount for Access" on my cell ph bill, but my sister does not. This is the 2nd month in a row I've received the discount. Does anyone know why I receive the discount, what it's for and why she doesn't receive the $15.00 credit / discount?
Thank you!