MTGE is still, in terms of SPO turns, fairly young. AGNC has ten times as many shares out. And it's been a long time since anyone did a SPO. So it's reasonable MTGE would play catch-up.
The "long time" part is interesting. It makes me think AGNC will want to do a mega-SPO as well. On their old schedule they'd be at nearly 600M shares out, and they're at 340M. Lots of room to reach for the missed bus.
You're right. The e-mail I got had "AGNC: American Capital Mortgage Investment Corp. Announces Public Offering Of Common Stock" as the subject line with the link to the PR Newswire release. The release itself is clearly about MTGE.