I don’t know if there is one answer to this question my friend, I had a conversation with a fb employee and I ask him the same thing he told me that audience size is a big factor to keep your numbers high also creatives that win on the auction is a big factor as well, frequency and many more factors can affect the performance of your campaign, I had CBO’s that I increase the budget x2 and x3 in one day meaning from 30€ to 60€ and to 90€ the same day and they worked fine and others increased a couple of € and got messed up, hope that helps ✌️