Here my doubt is that why is the naming not started from the carbon numbered 2 here as then the 1st Methyl group will be on the 2nd position , 2nd will be on the 3rd position and 3rd will be on the 5th position which will lead to the answer as :- 

2,3,5-Trimethylcyclohexene.

And why is this answer incorrect? please explain in detail.

If you see at the organic compound, there is a double bond.

As per the IUPAC nomenclature, first priority while numbering  is given to the double bond then the methyl group.

This is done so as to give the double bond the lowest numbering while naming the organic compound.

So the numbering will start from the double and the IUPAC name of the compound is 1,4,6- trimethylcyclhex-1-ene.

Overhere if you see the double bond and the methyl group is getting the lowest numbering, so the correct name of the organic compound is 1,4,6- trimethylcyclhex-1-ene.

If you start numbering from the carbon number 2, the methyl group will get number 2 and the name of the compound will be 2,3,5-Trimethylcyclohexene which is not correct.

Please keep in mind while numbering give priority to the functional group then the methyl group.

Hope the explanation helps you.