Turnover on service tax

Hello, I am travel agent. We charge Service tax based in abatements. For example 4.5% for packages, 1.5% on hotel booking like this. We have raised invoice worth apporx 20 lakhs (Exclusive of taxes) in the last financial year. However our turnover based on service tax returns comes a lot less. Is it correct in our field ? Will 20 lakhs be taken as trunover or based on service tax turnwover will be taken as final. Turnover based on service tax coming quite less as we do not charge full service tax, we charge it based on abatement. 

Please advice...