That is right. The HO may not have found the sitter they wanted.
I had a response from one HO after I had submitted an application that they were aware that I would be applying for more sits and that the chance of missing out of me would be there. I reply to every message and if I withdraw, I let them know why (other sit or time not right etc)
I am not saying that HO’s are to be blamed for the 7+ who may not be suitable. Waiting is required as long as both parties realise that by doing this, they may miss out on the opportunity to sit.
Sad that you had 6 applications and none of them were what you were looking for. In those situations, of course, wait until the right one turns up!