Generally, surface mould on walls is caused when the warm, moist air meets a cold surface to dew on.
Despite a fan which shifts 280m3 of water an hour and by leaving the bathroom window ajar (locked) when not in there, we still get light amounts of water on the wall. In the past, this would eventually turn to black mould spores, however after using the Ronseal stuff (probably same as Keithmac) it's not been able to grow. I should imagine without our ventilation efforts it would form regardless of the paint used.
Plan one day is to tank the whole bathroom!