HUK24: Contribution accounting more expensive than in the online offer

Hello, I have insured at the HUK24 a BMW convertible with VK (1000 Euro SB) and TK (150 Euro SB) with SF 22 for 550 Euro annual fee (invoice for 2009). Now for comparison exactly the same data that my contract has entered into the online offer invoice of HUK24 and as a result receives an annual fee of 423 Euro. That is at least over 20% less. Does any of you have an explanation? I have requested the HUK accordingly, but no answer yet.