For a brief window of time, third-party data site SteamSpy was absolutely 100 per cent accurate.
That's according to founder Sergey Galyonkin, who told PCGamesInsider.biz that using an achievement-based algorithm made by The End is Nigh Tyler Glaiel, his site was completely spot on with its numbers.
This workaround was quickly blocked by Valve.
"When we were using Tyler Glaiel's algorithm SteamSpy was 100 per cent accurate," Galyonkin said.
"It was accurate down to a single copy. I essentially rounded up numbers because I wasn't sure how I felt about having actual sales figures instead of estimates. But then it was accurate down to the copy. Several days later, they killed that again. I feel like they were misleading people. I don't feel that SteamSpy and accuracy was the problem. Accuracy was the problem."
Since this hole was plugged, however, Galyonkin has released the data scraped from the Steam database to ArsTechnica, which has published the top 1000 selling Steam games of all time as of July 1st, 2018.
A reminder: This data is extrapolated via achievement data from Steam's platform.
Heading the pack, unsurprisingly, is Team Fortress 2, which has sold 50.1m, ahead of Counter-Strike: Global Offensive which has shifted 46.3m.
Playerunknown's Battlegrounds has sold 36.6m copies since its March 2017 release date - considerably below the 42m which PCGamesInsider.biz estimated last week.
Fourth place goes to Unturned with 27.3m units sold, while Left 4 Dead 2 has sold a reported 23.1m.
Valve claims that it is working on its own data solution for Steam, and said in the past that the inaccuracies on SteamSpy were what concerned it. That excuse is wearing a bit thin now given that SteamSpy was - briefly - accurate and Valve closed off that method.
