There's no way DA:I sold more than Mass Effect 3. Maybe they mean launch numbers? Who knows, maybe i'm talking out of my ass. I loved DA:I but i had no idea it was that popular.
edit: Wait DA:I?? I hated that game lol. I thought we were talking about the original lol.
It sounds super super strange to me as well. From my experience though, and I might be talking out of my ass here as well, it seems like the dragon age series was super popular with women compared to mass effect that for some reason had less appeal so perhaps that's the difference? I honestly can't tell for sure.