I already broke down the sales figures to you on NeoGAF but to very quickly re-summarise the sales now in 2018, let's see...Actually reboot sold 11m copies (news from November last year).
Also can you post a source for TR2 numbers?
The report that TR 2013 allegedly sold 11m comes from this, an interview with Yosuke Matsuda (of Enix) that also made this report, which is an outdated and inaccurate representation of sales figures toward the TR franchise. For example, inside that .pdf, is this meme. Tomb Raider II has not sold 6.5 - 7m copies as the graph suggests.
So what does this mean?
Either Enix fudges sales numbers or that graph is inaccurate. Either way, both are sourced from Enix, so take as you will.
Using SteamSpy and this article from PCGamer, which is sourced from CD, according to him in 2015 Tomb Raider 2013 sold 8.5m copies. So using both of these sources it's safe to assume an educated guess that 2013 has sold between 8.5 - 10m copies as of 2018. It's more like 9m because SteamSpy has rounded a lot of games to the tenth by default.
Now, back in 2003, independent magazine publication GameState got a hold of the sales figures for many games including Tomb Raider II, which sold 8m on PS1 alone. This was 2003. This isn't including PC sales, which we can assume sold very well due to the game being sold out and re-released under different editions over the years, including the apt named 'Sold Out' publisher, which published PC games that sold very high numbers. Using educated guesses, we can assume the game must have sold around 1 - 2m copies on PC for Sold Out (and other publishers) to consider re-releasing Tomb Raider II alone.
That brings 2013 to 8.5 - 10m copies sold as of 2018, and Tomb Raider II around 8 - 10m copies sold as of 2003.
Now, include Steam, GOG, the re-release sales and PSN digital sales for Tomb Raider II. We know SteamSpy has put the game around 500k copies sold, GOG doesn't do sales but considering it's been on their service for years, it's common sense that it's sold at least a few hundred thousand copies on there. PSN as well. Retail re-releases probably didn't sell as much, so rounded up, that's an extra million or near enough to it.
That's a total of:
TR 2013 - 10m max (more like 9 because SteamSpy uses 10m to round up to with a lot of games)
TR II - 9 - 11m max (12m if we estimate that GOG + PSN sales are high as Steam)
That is a potential 2m difference in copies sold between 2013 and II.