User blog comment:Commdor/PC Gamer June 2011 - ME3 Info/@comment-2250460-20110506193342/@comment-217683-20110506193459

Maybe they found another ship and queen egg? Or, assuming the Reapers were the ones who provoked the rachni into going to war with the Citadel races, maybe the Reapers kept a few rachni as specimens.