Yes, it is a decent score and I'm part of the top 2 percent of my batch, so god alone knows. Was rejected at Oxbridge :( So you're suggesting I should seek clarification?
I think they have made a mistake. The condition you have been given differs markedly from what the UCL website suggests should be the case. If you go here:http://www.ucl.ac.uk/prospective-students/graduate/degrees/taught/tmllawsing12
And then click on the 'Application and Entry' tab, you can select India in a drop-down menu. It says that the entry requirements should be:
"Equivalent qualifications for India
The award of any of the following from a recognised institution:
A Bachelor's degree with an overall mark of 65% in the First Class/Division and above. The overall mark required is also dependent on the subject and the ranking of the institution at which the degree has been taken. The rankings published by India Today may be used as a guide."
It doesn't make much sense to me that you have been set a conditional offer 10 points above this mark. It does say that the mark is 'dependent on subject' but that is probably because the box is just 'grabbing' information from this page:http://www.ucl.ac.uk/prospective-students/international/countries/south-asia/india
and that page obviously doesn't know which subject you are interested in.
It also states that 'the ranking of the institution'... where you obtained your degree is relevant. However (a) I suspect that is not true, I went to a bottom-ranked English university and got a completely standard offer, (b) even if they do factor it in, and your university is also bottom-ranked according to 'India Today', a 10% increase in what is needed seems ridiculously high. I could see them offering, say, 68% instead of 65% in such a case but not 75% instead of 65%.
In summary, then, I think they have simply made a mistake with your offer condition and you should contact them perhaps drawing their attention to the pages I've linked to above.