It could very well be what Ancestry DNA refers to as statistical "noise," meaning that such a small trace region is very likely not in fact part of your ancestry but just that based on the samples they have here, you may share a genetic marker with someone from that region.
Personally, I have <1% South Asian DNA in my results. I don't have any immediate "paper" ancestors from India, Pakistan, or any other countries in the South Asia region (my entire ancestry is European/Mediterranean). Also of significance is the presence of overlap between different adjacent regions. South Asia tends to overlap with the Caucasus region if you look at the explanations given, and according to my results, 10% of my DNA came back as "Caucasus."