Why do iphone vodafone keep changing its pricing

do vodafone normally change their offers/$ value off phones! i am due to sign up for a 24 mth contract with vodafone as they offer the minutes/txt/voicemail that suits for business, and i was buying 2 contracts with a 4s, and a 4.as iphones were out of stock in the last week the rebate came to $545 for the 4s, and the iphone 4 changed from $399 to $699 in that time.as the iphone 4s is back in stock, the phone is $849 on 24 mth contract and the 4 has stayed at $699. frustrating.