PPF — wrong interest amount credited to ppf account

Hi i have opened ppf account on april 2017 at sbi bank at the rate of interest 7.8. I deposited 60k in the financial year 2017-18. If i calculate 7.6 rate of interest also whole year 4600 amount should be credited by i have received only 3300 as interest. If i approach sbi they neither cared about it but while opening acc thy will have lot about us. They said me to go and ask government. Don't know who to ask