Besides the fact that these numbers are educated guesses based on sample siezures, and weapons that present themselves and known legal sales to determine a number of privately possessed arms...
I think it is quite telling that after the USA, which is predominantly legal guns, several nations are listed with the highest totals where legal onwership is all but non-existant. Mexico has less than 5,000 legal weapon owners, it has ranged from around 2,000-4,000 for years, and the types are very limited and restricted. Yet they have tons of small arms illegaly possessed, meaning it is mainly criminals that have them.
China ranked 3rd yet has some of the strictest gun laws in the world, even giving the death penalty with ease to criminals with a firearm, or selling or providing such arms to people. So we are safe to assume all those guns making it the third highest in the world are held by criminals.
"A system of licensing and registration is the perfect device to deny gun ownership to the bourgeoisie." Vladimir Ilyich Lenin
In China, where the government controls some media outlets as government agencies and the media in general must operate within strict guidlines concurrent with government views, events like Virginia Tech are heavily broadcast repeatedly and discussed in a way to support existing arms laws. CCTV (China Central Telivison) an agency of the Chinese government for example gave it a lot of air time.
India ranked second has a serious martial history, self defense culture, etc, in fact carrying arms in the form of knives and swords is a part of the culture dating far back(before guns even existed). Yet it has rather draconian gun laws dating back to its British rulers, and later government distrust of its people, so it is safe to assume most of those are illegaly possessed.
So the study is interesting in more than one way.