Huh. I just moved from MD at the start of July and I did the exact same thing you did when I transferred my plates. I had it inspected with MD plates, and then took the inspection certificate to the DMV and they gave me a DC title, DL, and plates. Well, the title comes in the mail, but it was all taken care of at the DMV at one time.
You should probably call the DMV and explain that to ’em? Maybe they mistook your Feb inspection for an older one?