IIRC the brake fans cool the temperature sensors much faster than the actual brakes so the sensors underread. I think the idea is that if you've had the fans on for the duration of the turnaround and the brake temps are still indicating 150C then the chances are the brakes are actually at 300C so you should delay takeoff. This may be a typo as our manuals stated the limits as 300C fans off or 150C fans on, but it's still a bit of a muddle.