Income tax notice under section 143(1), incorrect tds data

I have received Income tax demand notice whereby AO has calculated 2,39,097 as TDS while form 26AS shows 2,69,476 as TDS. The difference along with interest has been demanded from my side.There is a mistake on Income Tax department side.

How can a error happen between 26AS and data for tax computation from In come Tax Department side?.

I have disagreed with the demand notice in the Incometaxefiling site.

What else should I do now or should I wait for AO to reply?