If your question is asked correctly and your earnings did drop down, then I'm very sure that there must be a reason for that to happen. Even though I'm still new to the site, I have some people whom I know to have been here for a long time and none of them have experienced such problems in the past with their earnings being cut down.
What I'm trying to say is that maybe you have breached some of the forum rules and regulations with posting which led the administrative team to cut down your earning when you request for withdrawal. I have seen this kind of thing happened in some other sites. When you send for payment withdrawal, your account would be checked as well as your posts before the payment would be approved. So, when there are irregularities in your account, it's bound to affect your pay.