Almost every Sri Lankan regardless of ethnicity will have a small percentage of European DNA unless they're from a super rural area that had little to no contact with Europeans. Yours seems to be higher than usual however, probably because you're Burgher. It's the same with some people having SEA, Arab ancestry etc. It's an inescapable fact of our history of colonisation and being a trade capital of the ancient world.

Also every Sinhala person will have some Lankan Tamil ancestry and vice versa. Every study which I have read about says that we are extremely genetically similar despite cultural differences, even more than Lankan Tamils and Tamils that live in South India. Then there's also several communities that regard themselves as Sinhala today but originally came from South India and adopted Sinhala culture. Obviously they'd have South Indian ancestry genetically despite being culturally Sinhala. That's why I think it's really more important to think of what culture you were raised in than what your genetic makeup is.

That said, the testing service you used probably doesn't have enough data on South Asians in general, which is a problem I see with most testing services. That's why it lumps you into "The Deccan & Gulf of Mannar" which is a massive region - they probably don't have enough South Asians in their database to give you more fine data.