Because that was the last meeting they could have before Colossus required Forbin to be under 24/7 surveillance. I think it makes perfect sense. How else could

Because that was the last meeting they could have before Colossus required Forbin to be under 24/7 surveillance. I think it makes perfect sense. How else could he send/get information unless he had a plan to convince Colossus to not observe him. My big issue with this whole thing is that they actually believed Colossus wouldn't listen in because it said so.