Vat related query

DEAR SENIORS

an contractors,While prepareing contract invoice VAT is considered only 75% of the goods why? for eg: goods of VAT 12.5% & 5% used on contract, BUT while raising invoice only 75% of 12.5 & 5% ie;9.37 & 3.67% is considered and also service tax of 4% is levied .................WHY ???   Why they are not raising contract invoice of full 12.5 / 5% VAT on item Used on contract...... kindly explain me???