It boils down to why they kept a journal or notes. If someone keeps it for themselves they can be more honest and more telling. Those creating something for public consumption will be careful in what they write and could even be writing something with the intention to give a false impression. If they were writing it for public consumption they would not have wanted to reveal their disdain for Sheila or Jeremy so it supports them creating such for their own private use.
How would the diaries become available to the public ? There was no internet in 1985.
I suppose RB could sell them to the press. He hasn't done this. Surely this couldn't be done pre verdict. So really there was no benefit in lying in them.
As RB said, the police suggested he put his thoughts and actions down in diary form prior to compiling his WS. Perhaps this is procedure to help witnesses remember things.