Idea Cellular Ltd. — I have received a massage Special full talktime officer

I have received a message from IA-501 full on[protected] @1:29:04 PM { Special Full talktime officer recharge with Rs. 501/- and get Rs.501/- Full talk time. This officer is only for you for few days} after I made a call to 53543 - it says the validity for next two days only. I went to Retailer on[protected] @ around 10.30 AM and recharge with Rs. 501/- I got only Rs. 455/- some thing. What is the problem. Plese verify and justify to me.