Perhaps Betfair has no memory but the punters do - short term memory.
My studies show when a horror run of outs on favourites occurs, the prices get longer and longer and hence the overlays occur.
Just as when they romp home 50% of the day, they get shorter and shorter as success continues.
Why?
Because either favourite backers are cashed up and think this is the day, or they simply get frightened and back off so they have cash for another day.
Might seem all paranoia and conjecture, but looking at the prices (and I'm talking about heavily fancied horses) the pattern does emerge.
Self doubt, the conspiracy theories, the "perhaps I got it wrong today with the ratings" etc etc, all have an impact on how much is wagered.
As a wagering group, punters have the shortest memories of any group, including bookies
Some of my best days have been where only one favourite gets up for the day (and I bet level stakes) and the overlay goes to 200% plus.