Yep, that has become quite common. DL will severely overbook coach when business is fairly available. You gotta take the plane as a whole. So for example there's a certain A330-200 that's heading overseas today with 16 open in business, but -17 in coach. The plane is overbooked by 1. If absolutely everybody shows up, there will be 17 HKs without assigned seats in coach, and the agent will accomodate as many in business as he can rather than buy off or deny boarding to all of them. When looking at the airport standby list, you gotta take those HKs into account.

For what it's worth, I use a rule of 12 at hub airports. If the number of overbookings for the plane + nonrevs ahead of me ≤ 12 (as of, say, 3 hours prior), I usually get on. At non hub airports the number is less because no misconnects. The euros in particular are very good at making their flights, I've found the number of no-shows at european airports other than AMS & CDG is usually 2 or 3.