There is a new theory by an analyst, as the US goverment, or at least the law enforcement divisions are accumulating the seized BTC. No direct evidence, of course, but there are so much signs, what suggest this thing.
Officer write the reports long time after the actions. Many reports also does not exist in digital, only on paper. There is a massive lack of transparency. Once a few coins just revealed as seized by authorities, it can be tracked. But if they catch a criminal wallet, and doesnt change anything, we simple cant know, how much BTC the government have.