Battery time in Dell Vostro 3520 and bad support

Hi. I bought Dell Vostro 3520 in beginning of March and it's battery was already weaker than manufacturer declares (I think it should be 4-6 hours, no?). Currently it lasts only 2,5-3 hours. I have 4 years pro-support warranty (but in reality it's not that "Pro") and they asked me to send a generated battery report which shows some <Profanity is never needed nor allowed on our Forum. DELL-Admin> mWh parameter which they claim is still OK and so there is no basis for replacement. I don't think this is fair. What is your experience with Dell warranty ?