Is there some headcounting going on here?

On May 4th, 2005 the First Circuit sitting en banc heard argument in Igartúa de la Rosa v. United States, No. 04-2186 (1st Cir. Oct. 14, 2004). The 3 judge panel opinion was withdrawn when petitioners' requested rehearing or, alternatively rehearing en banc, and the panel agreed to rehear the case and vacated its opinion and judgment and ordered additional briefing by the parties. With briefing completed, but before rehearing by the 3 judge panel, the en banc Court decided to take up the case. A link to the audio of the en banc oral argument can be found here.

Today the First Circuit has issued a Memorandum and Order (per curiam) with a strong dissent by Circuit Judge Juan R. Torruella, over whether Senior Circuit Judge Levin Campbell, who sat in the initial 3 judge panel, can sit on the en banc panel. The majority concludes that he may, and Judge Torruella argues that he may not. It is abundantly clear that had the Court taken up the matter en banc following the original panel's opinion, Senior Judge Campbell would be qualified to sit on the en banc panel. But Judge Torruella argues (and I'm simplifying a bit here) that because the panel's opinion and judgment had been vacated by the 3 judge panel itself and no rehearing before the panel had taken place when the en banc Court motu propio decided to take over, the en banc Court is really reviewing the district court's opinion and not the opinion by a 3 judge panel, since there was no longer an opinion by the panel at the time of the en banc court's decision to take the case, and in that scenario the Senior Judge cannot sit on the en banc.

Aside from the legal niceties, I wonder whether there is some counting going on here, or merely a discussion of purely legal issues. After all, there are only 6 active Judges in the First Circuit, and Judge Torruella was the dissenting judge in the original panel's decision. Hey! It happens in the Supreme Court, so why would it not happen elsewhere?