Withholding tax code - not calculating exact tax amount

Dear Gurus, We have created a tax code to calculate TDS 1% ( 194 c ) . In fb60 it works fine and calculating correctly but for only one vendor it calculates Rs. 27 less and for another vendor it calculates in excess. I checked the all the parameters set for the vendor in vendor master it is exactly same with other vendors . Also I checked the ledger account it also correct . What else I have to check to rectify this error.

best regards

selvakumar.n.