Correct IDs are (mapAreaId/instanceId):
3698/559 = Ruins of Lordaeron
3702/562 = Blade's Edge Arena
3968/572 = Nagrand Arena
Parasite IDs in my case are:
530 which is the instanceId of Outlands (10 occurrences out of 2355 matches)
1 which is the instanceId of Kalimdor (4 occurrences out of 2355 matches)
Those occurrences seem to match times where I was queuing in Shattrath and in Darnassus, respectively.
So it becomes quite clear: this seems to be a race condition where the map ID is computed after leaving the arena. Not sure we can solve this, and anyway it doesn't seem to happen too often.