Do they not have calculators? I remember one behind the scenes article saying they rushed candidates to try and force as many mistakes out of them. Like they stopped the main team communicating with the sub team, so any errors were locked in.
Karen told the girls about the fish error, so presumably they couldn't change it after. Karen can't "interfere", but she'll happily bait them with bad news for good TV.
I'm a firm believer that edits are done to make them look as stupid as possible. The alternative is that the candidates don't watch the show before applying, and nobody wanted to be team leader for the 1st task, knowing that they usually go home.