the reporting period doesn't end at the end of the month, so even though DR2 came out on the 28th it still had 5 days. It will probably add another 250-300k this month I'd guess, but who knows how long it will continue to sell. I think 2.5-3 million is unrealistic as well, but Capcom apparently didn't.
Guitar Hero might sell well over time because it has tons of casual appeal and people aren't going to run out and buy it day one, but that series has still fallen harder than anything this side of Tony Hawk.