I think Charlie's logic was that Sam and Sophie were unlikely to be nominated by the other housemates or voted out by the public.
If they sent 2 other housemates to the safe house and kept Sam and Sophie with the remaining unsafe folk, the remaining unsafe folk had a greater chance of being nominated then evicted.
Ergo, take Sam and Sophie out of the equation and all the remaining "unsafe" housemates are on a more level playing field.
This is only logical of course, if you agree with her theory in the first place.
I thought she did very well to think that through in the short time they had before they had to come up with an answer.
I thought that Dan was a bit miffed but there wasn't enough time to argue about it.
And, of course, when all is revealed, her logic will come back to bite her on the bum and possibly make her a target for the following week.